Add README and CHANGELOG for functions_framework_tool package (#144)

* Add README and CHANGELOG for functions_framework_tool package

* Minor nits

Author: subfuzion

functions_framework_tool/CHANGELOG.md

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+# Changelog
+
+## 0.3.0
+
+The first version is `0.3.0` to align with the rest of published Functions
+Framework packages. This package is intended to support tools for working with
+Functions Framework projects. In this early iteration, a CLI tool (`dartfn`) is
+provided with a `generate` command for scaffolding new projects into an empty
+directory. Three initial generator templates are available (`helloworld`,
+`json`, and `cloudevent`).

functions_framework_tool/README.md

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+# Functions Framework Tool package for Dart
+
+This package is intended to support tools for working with Functions Framework
+projects.
+
+In this early iteration, a CLI tool (`dartfn`) is provided with a `generate`
+command for scaffolding new projects into an empty directory. Three initial
+generator templates are available:
+
+* helloworld - a basic HTTP handler function
+* json - a simple function handler that accepts and sends JSON
+* cloudevent - a simple cloudevent function handler
+
+To install the `dartfn` tool on your machine, run the following command:
+
+```shell
+dart pub global activate dartfn
+```
+
+Run `dartfn` without any arguments to confirm it is installed.
+
+```shell
+dartfn is a tool for managing Dart Functions-as-a-Service (FaaS) projects.
+
+Usage: dartfn <command> [arguments]
+
+Global options:
+-h, --help    Print this usage information.
+
+Available commands:
+  generate   Generate a project to get started.
+  version    Print the current version.
+
+Run "dartfn help <command>" for more information about a command.
+```
+
+## List available generators
+
+```shell
+dartfn generate --list
+```
+
+Output
+```text
+Available generators:
+cloudevent - A sample Functions Framework project for handling a cloudevent.
+helloworld - A sample "Hello, World!" Functions Framework project.
+json       - A sample Functions Framework project for handling JSON.
+```
+
+If you want the generator list as JSON, use the hidden `--machine` flag:
+
+```shell
+dartfn generate --machine
+```
+Output
+```json
+[{"name":"cloudevent","label":"Dart Package","description":"A sample Functions Framework project for handling a cloudevent.","categories":["dart"],"entrypoint":"bin/server.dart"},{"name":"helloworld","label":"Dart Package","description":"A sample \"Hello, World!\" Functions Framework project.","categories":["dart"],"entrypoint":"bin/server.dart"},{"name":"json","label":"Dart Package","description":"A sample Functions Framework project for handling JSON.","categories":["dart"],"entrypoint":"bin/server.dart"}]
+```
+
+## Run a generator
+
+```shell
+mkdir demo
+cd demo
+dartfn generate helloworld
+```
+
+Output
+```text
+project: demo
+Creating helloworld application `ex`:
+  demo/.gitignore
+  demo/Dockerfile
+  demo/README.md
+  demo/analysis_options.yaml
+  demo/bin/server.dart
+  demo/lib/functions.dart
+  demo/pubspec.yaml
+  demo/test/function_test.dart
+8 files written.
+
+--> to provision required packages, run 'pub get'
+```
+
+## Print the version and check for updates
+
+```shell
+dartfn version
+```
+
+Output
+```text
+Google Functions Framework for Dart CLI (dartfn) version: 0.3.0
+```
+
+If a newer version is available, the command will inform  you. Example:
+
+```shell
+dartfn version
+```
+
+Output
+```text
+Google Functions Framework for Dart CLI (dartfn) version: 0.2.0
+Version 0.3.0 is available! To update to this version, run: `pub global activate dartfn`.
+```
+
+If you just want the version number, use the `-s` or `--short` option:
+
+```shell
+dartfn version --short
+```
+
+Output
+```text
+0.3.0-dev
+```