📝 Add installation and usage instructions (#17)

Author: paduszyk

README.md

@@ -17,6 +17,182 @@ as a helper in projects related to version control systems, versioning, and
 automatic changelog generation. Applications in automation tools for validating
 commit and pull request messages seem feasible as well.
 
+## Features
+
+- Handle individual Gitmojis and their lists using Python classes. 👔
+- Fetch Gitmoji data directly from the official [Gitmoji API][gitmoji-api]. 😜
+- Graceful degradation: If the API is unavailable, fall back to backup data. 🛟
+
+## Installation
+
+It is recommended to install the package directly from PyPI using `pip`:
+
+```console
+$ pip install python-gitmojis
+```
+
+or any other dependency manager of your preference. After installation, the
+package functionalities can be accessed by importing them from the `gitmojis`
+module:
+
+```python
+import gitmojis
+```
+
+## Usage
+
+### Data model
+
+The data model incorporates two classes representing individual Gitmojis and
+their collections ("guides"), namely, `Gitmoji` and `Guide`, respectively.
+
+The classes are defined in `gitmojis.model`, but can also be accessed directly
+from `gitmojis` as well:
+
+```python
+from gitmojis import Gitmoji, Guide
+```
+
+#### `gitmojis.model.Gitmoji`
+
+The `Gitmoji` class is a Python `@dataclass` ([PEP 557][PEP-557]) that uses a
+set of fields consistent with the [JSON schema][gitmoji-schema] proposed in the
+original Gitmoji project, namely:
+
+- `emoji: str` – the emoji character representing the Gitmoji;
+- `entity: str` – the HTML entity of the Gitmoji;
+- `code: str` – the emoji's code to be used in rendering Gitmojis in the
+  Markdown documents;
+- `description: str` – a short note on the type of changes represented by
+  the commits or pull requests marked by the Gitmoji;
+- `name: str` – a text identifier of the Gitmoji;
+- `semver: str | None` – the [Semantic Versioning](https://semver.org)
+  level affected by the changes marked with the Gitmoji; the allowed string
+  values are `"major"`, `"minor"`, and `"patch"`.
+
+The class can be used to create immutable objects representing individual
+Gitmojis, for example:
+
+```python
+from gitmojis import Gitmoji
+
+gitmoji = Gitmoji(
+    emoji="🤖",
+    entity="&#x1f916",
+    code=":robot:",
+    description="Add or update Dependabot configuration.",
+    name="robot",
+    semver=None,
+)
+
+assert gitmoji.emoji == "🤖"
+```
+
+> Note that when creating a new `Gitmoji` instance, all the `@dataclass` fields
+> are required. Furthermore, they all must be passed as keyword arguments.
+
+#### `gitmojis.model.Guide`
+
+The `Guide` class aims to provide a custom list-like structure to manage a
+collection of Gitmojis. Its instances are created simply by passing an iterable
+of `Gitmoji` objects (as the `gitmojis` keyword argument) to the class
+constructor:
+
+```python
+from gitmojis import Gitmoji, Guide
+
+gitmojis_json = [
+    {
+        "emoji" : "🤖",
+        "entity" : "&#x1f916",
+        "code" : ":robot:",
+        "description" : "Add or update Dependabot configuration.",
+        "name" : "robot",
+        "semver" : None,
+    },
+    # ...
+]
+
+guide = Guide(
+    gitmojis=[Gitmoji(**gitmoji_json) for gitmoji_json in gitmojis_json]
+)
+
+assert guide[0].emoji == "🤖"
+```
+
+The class is based on `collections.UserList`. Currently, it doesn't override
+any base class methods nor does it implement any custom functionality.
+
+### Fetching Gitmojis from the API
+
+The main package functionality is implemented as a plain Python function, named
+`fetch_guide`. It can be imported either from `gitmojis` or directly from its
+source, i.e. the `gitmojis.core` module.
+
+Usage of the function is extremely easy. In the simplest case, it can be called
+without any arguments:
+
+```python
+from gitmojis import fetch_guide
+
+guide = fetch_guide()
+```
+
+The function uses the `requests` library to return a `Guide` instance containing
+the current state of the official [Gitmoji API][gitmoji-api]. If the API is
+inaccessible, the guide can be populated using the data retrieved from the local
+backup file. Such behavior can be triggered by passing `True` as the value of
+the `use_backup` keyword argument (it defaults to `False`):
+
+```python
+from gitmojis import fetch_guide
+
+guide = fetch_guide(use_backup=True)  # will work even if you're offline
+```
+
+### Command-line interface (CLI)
+
+The package comes with a simple CLI which can be run using the `gitmojis`
+command:
+
+```console
+$ gitmojis
+Usage: gitmojis [OPTIONS] COMMAND [ARGS]...
+
+  Command-line interface for managing the official Gitmoji guide.
+
+Options:
+  --use-backup  Use the backup to fetch data if the API request fails.
+  --version     Show the version and exit.
+  --help        Show this message and exit.
+
+Commands:
+  sync  Synchronize the backup file with the current state of the API.
+```
+
+As seen, currently the CLI provides only the `sync` command which can be used
+to update the Gitmoji data backup file to the current state of the official API
+endpoint.
+
+> Checking for the updates of the API state is compared to the backup file by
+> executing the `sync` command at GitHub Actions runner every week. The
+> respective workflow automatically applies the updates and opens a pull
+> request introducing them to the codebase. We plan to do the version bump (on
+> a patch level) upon merging each of such pull requests. Therefore, to stay
+> tuned with the Gitmoji API backed up by this library, you should update
+> the package systematically. This particularly concerns the developers, who
+> work with local repositories most of the time.
+
+## Contributing
+
+This project is open-source and embraces contributions of all types. For
+comprehensive instructions on how to contribute to the project, please refer to
+our [Contributing Guide][contributing-guide].
+
+We require all contributors to adhere to our [Code of Conduct][code-of-conduct].
+While it may seem intricate at first glance, the essence is simple: treat
+everyone with kindness! 🙂
+
 ## Credits
 
 The idea of Gitmoji was originally proposed, developed, and maintained by
@@ -33,14 +209,19 @@ Created by Kamil Paduszyński ([@paduszyk][github-paduszyk]).
 Released under the [MIT License][license].
 
 [black]: https://github.com/psf/black
+[code-of-conduct]: https://github.com/paduszyk/python-gitmojis/blob/main/.github/CODE_OF_CONDUCT.md
 [codecov]: https://app.codecov.io/gh/paduszyk/python-gitmojis
+[contributing-guide]: https://github.com/paduszyk/python-gitmojis/blob/main/.github/CONTRIBUTING.md
 [github-build-workflow]: https://github.com/paduszyk/python-gitmojis/actions/workflows/build-package.yml
 [github-carlosquesta]: https://github.com/carloscuesta
 [github-paduszyk]: https://github.com/paduszyk
+[gitmoji-api]: https://github.com/carloscuesta/gitmoji/tree/master/packages/gitmojis#api
 [gitmoji-repository]: https://github.com/carloscuesta/gitmoji
+[gitmoji-schema]: https://github.com/carloscuesta/gitmoji/blob/master/packages/gitmojis/src/schema.json
 [gitmoji-website]: https://gitmoji.dev
 [license]: https://github.com/paduszyk/python-gitmojis/blob/main/LICENSE
 [mypy]: https://github.com/python/mypy
 [nox]: https://github.com/wntrblm/nox
+[pep-557]: https://peps.python.org/pep-0557/
 [pre-commit.ci]: https://results.pre-commit.ci/latest/github/paduszyk/python-gitmojis/main
 [ruff]: https://github.com/astral-sh/ruff