Add usage information

Author: PeterJCLaw

README.md

@@ -1,3 +1,24 @@
 # SRComp Validate Action
 
 A GitHub Action to validate a compstate.
+
+## Usage
+
+This action aims to be a fully self-contained validator for Compstate repos. Usage is therefore very simple:
+
+``` yaml
+name: Validate
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: PeterJCLaw/[email protected]
+```