Flesh out README a bit

README.md

@@ -1,2 +1,29 @@
 # swiftly-action
-A GitHub Action for using the Swiftly toolchain manager
+
+A GitHub Action for installing the [Swiftly toolchain manager for Swift](https://swift-server.github.io/swiftly/)).
+
+## Usage
+
+To use Swiftly in your workflow, specify the repository followed by a tag as with any action:
+
+```yaml
+steps:
+  - uses: actions/checkout@v4
+  - uses: vapor/swiftly-action@v0.1
+    with:
+      toolchain: latest # optional
+```
+
+On completion, `swiftly`'s executable will be present in `PATH`, and the `SWIFTLY_HOME_DIR` and `SWIFTLY_BIN_DIR` environment variables will be available to all following steps.
+
+## Arguments
+
+At this time, the only argument supported is `toolchain`, which specifies a Swift toolchain name for Swiftly to install along with Swiftly itself. If the argument is omitted, only Swiftly itself is installed.
+
+## Contributing
+
+Contributions are welcome! Check out Vapor's [Contribution Guide](https://github.com/vapor/.github/blob/main/CONTRIBUTING.md).
+
+## License
+
+All code in this repository is released under the [MIT License](https://github.com/vapor/swiftly-action/blob/main/LICENSE).