add some docs to the readme

README.md

@@ -3,3 +3,17 @@
 ![github actions badge](https://github.com/icewind1991/attic-action/workflows/attic-action%20test/badge.svg)
 
 Github action to use [attic](https://github.com/zhaofengli/attic) for caching.
+
+## Usage
+
+- Setup an attic cache [as normal](https://docs.attic.rs/tutorial.html)
+- Add your attic auth token as a secret to your repo
+- Add a step to your ci:
+
+```yaml
+- uses: icewind1991/attic-action@v1
+  with:
+    name: cache-name
+    instance: https://cache.example.com
+    authToken: '${{ secrets.ATTIC_TOKEN }}'
+```