docs: update readme with usage

Author: robjtede

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+    "cSpell.words": [
+        "msrv"
+    ]
+}

README.md

@@ -1,2 +1,27 @@
-# msrv
-Read MSRV from package or workspace manifest
+# MSRV
+
+> Read MSRV from package or workspace manifest.
+
+## Usage
+
+```yml
+jobs:
+  read_msrv:
+    name: Read MSRV
+    uses: actions-rust-lang/msrv/.github/workflows/msrv.yml@main
+
+  build_and_test:
+    needs:
+      - read_msrv
+
+    strategy:
+      fail-fast: false
+      matrix:
+        version:
+          - { name: msrv, version: "${{ needs.read_msrv.outputs.msrv }}" }
+          - { name: stable, version: stable }
+```
+
+## Drawbacks
+
+- Assumes all packages in workspace have same MSRV.