Update documentation

Add readme file.
Update description.

Author: alexesprit

LICENSE.md

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 The MIT License (MIT)
+
 Copyright (c) 2019 alexesprit
 
 Permission is hereby granted, free of charge, to any person obtaining a copy

README.md

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+## action-update-file
+
+Update (i.e. commit and push) a given file on GitHub.
+
+## Usage
+
+The action requires GitHub token; no username or e-mail are required.
+
+```yml
+name: Resources
+on: repository_dispatch
+jobs:
+  resources:
+    name: Update resources
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/setup-node@v1
+    - name: Update resources
+      uses: alexesprit/[email protected]
+      with:
+        file-path: path/to/file
+        commit-msg: Update resources
+        github-token: ${{ secrets.GITHUB_TOKEN }}
+```
+
+## License
+
+See the [license file][License].
+
+[License]: https://github.com/alexesprit/action-update-file/blob/master/LICENSE.md

action.yml

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 name: Update file on GitHub
-description: Update (i.e. commit and push) a given file
+description: Update (i.e. commit and push) a given file on GitHub
 author: alexesprit
 inputs:
   # Required