Add README

README.md

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+# Latest tag
+
+Automatically creates & updates a `latest` tag pointing to your latest release.
+
+When using GitHub Actions you always have to put a reference for every action you use in your worflows: that means that you either need to choose a specific version or you need to use a branch.  
+If you want to use the **latest** release of an action you can only hope authors are mantaining a `latest` tag that they update with every version: although not impossible, it's not that easy to find someone willing to do that.
+
+That's why I made this action: if you're the kind of guy that doesn't like to update tags you can simply use this action and forget about it. You can just put `latest` in the documentation: your users will get the benefits of using a branch as ref and the security of using only stable versions.
+
+## Usage
+
+You can use a workflow like this:
+
+```yaml
+name: Add latest tag to new release
+on:
+  release:
+    types: [published] # This makes it run only when a new released is published
+
+jobs: 
+  run:
+    name: Add/update tag to new release
+    runs-on: ubuntu-latest
+
+    steps: 
+    - name: Checkout repository
+      uses: actions/checkout@master
+
+    - name: Run latest-tag
+      uses: EndBug/latest-tag@latest
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # Leave this line unchanged
+```
+
+### Environment variables:
+
+The only `env` variable required is the token for the action to run: GitHub generates one automatically, but you need to pass it through `env` to make it available to actions. You can find more about `GITHUB_TOKEN` [here](https://help.github.com/en/articles/virtual-environments-for-github-actions#github_token-secret).  
+With that said, you can just copy the example line and don't worry about it. If you do want to use a different token you can pass that in, but I wouldn't see any possible advantage in doing so.
+
+## License
+
+This action is distributed under the MIT license, check the [license](LICENSE) for more info.