docs(readme): add basic readme

README.md

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+# release-please-action
+
+Composite [GitHub Action][1] which wraps [release-please-action][2] and
+[github-app-token][3] actions, with some opinionated default settings.
+
+[1]: https://github.com/features/actions
+[2]: https://github.com/google-github-actions/release-please-action
+[3]: https://github.com/tibdex/github-app-token
+
+## Features
+
+- Makes it easy to run release-please as a GitHub App.
+- Defaults to placing release place config and manifests within the `.github`
+  folder in the repository root:
+  - `.release-please-manifest.json` -> `.github/release-please-manifest.json`
+  - `release-please-config.json` -> `.github/release-please-config.json`