Add a README for the GitHub Action

Author: webknjaz

README.md

@@ -1,2 +1,37 @@
-# pypi-action
-This is a GitHub Action
+# PyPI publish GitHub Action
+This action allows you to upload your [Python distribution package](
+https://packaging.python.org/glossary/#term-distribution-package) to
+PYPI.
+
+
+## Usage
+To use the action simply add the following lines in the end of your
+`.github/main.workflow`.
+
+```hcl
+action "Upload Python dist to PyPI" {
+  uses = "re-actors/pypi-action@master"
+  env = {
+    TWINE_USERNAME = "f'{your_project}-bot'"
+  }
+  secrets = ["TWINE_PASSWORD"]
+}
+```
+
+N.B. Use a valid tag, or branch, or commit SHA instead
+of `master` to pin the action to use a specific version of it.
+
+
+### Environment Variables and Secrets
+- **`TWINE_USERNAME`**: set this one to the username used to authenticate
+against PyPI. _It is recommended to have a separate user account like
+`f'{your_project}-bot'` having the lowest privileges possible on your
+target dist page._
+- **`TWINE_PASSWORD`**: it's a password for the account used in
+`TWINE_USERNAME` env var. **ATTENTION! WARNING! When adding this value
+to the Action node in your workflow, use SECRETS, not normal env vars.**
+
+
+## License
+The Dockerfile and associated scripts and documentation in this project
+are released under the [GPL v3](LICENSE).