chore: Add documentation for using Dafny setup action in CI

docs/DafnySetupCI.md

@@ -0,0 +1,40 @@
+# Using Dafny in GitHub Actions
+
+Use the [setup-dafny-action](https://github.com/dafny-lang/setup-dafny-action) to install Dafny in your CI workflows.
+
+## Basic Setup
+
+```yaml
+- name: "Install Dafny"
+  uses: dafny-lang/setup-dafny-action@v1
+```
+
+## Specific Version
+
+```yaml
+- name: "Install Dafny"
+  uses: dafny-lang/setup-dafny-action@v1
+  with:
+    dafny-version: "4.9.1"
+```
+
+## Nightly Build
+
+```yaml
+- name: "Install Dafny"
+  uses: dafny-lang/setup-dafny-action@v1
+  with:
+    dafny-version: "nightly-latest"
+```
+
+## Build from Source
+
+```yaml
+- name: "Install Dafny"
+  uses: dafny-lang/setup-dafny-action@v1
+  with:
+    dafny-version: "4.9.1"
+    build-from-source: "branch-name"
+```
+
+The action sets `DAFNY_VERSION` environment variable and works on all platforms including macOS.