Add usage section to readme

README.md

@@ -1 +1,32 @@
 # setup-roc
+
+A GitHub Action to install Roc.
+
+## Usage
+
+Add the following step to a GitHub actions workflow file to install Roc.
+
+```yaml
+steps:
+  - name: Install Roc
+    uses: hasnep/setup-roc@main
+    with:
+      roc-version: nightly
+```
+
+### Inputs
+
+- `roc-version` - The version of Roc to use, defaults to `nightly`.
+  Mutually exclusive with the `roc-version-file` input.
+- `roc-version-file` - Path to the file containing the Roc version to use.
+  Mutually exclusive with the `roc-version` input.
+- `token` - The token used to authenticate when fetching from GitHub.
+
+### Outputs
+
+- `roc-version` - The installed Roc version.
+- `roc-path` - The absolute path to the Roc executable.
+
+## Licence
+
+This repository is released under the [MIT licence](./LICENCE) and is based on the [actions/setup-python](https://github.com/actions/setup-python) repository which is also released under the [MIT licence](./LICENCE-actions-setup-python).