Add more docs on gha

docs/src/github_actions.md

@@ -1,6 +1,37 @@
 # GitHub Actions
 
-To set up a formatting check in CI, you can use [the `julia-actions/julia-format` action](https://github.com/julia-actions/julia-format): please see that repository's documentations for instructions on how to set it up.
+To set up a formatting check in CI, here is a barebones action.
+This will just fail if any files are not formatted.
+
+!!! note "Pin the version of JuliaFormatter"
+    It is **strongly** recommended that you pin the version of JuliaFormatter used in your CI workflow.
+
+```yaml
+name: Format
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  format:
+    name: Format
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # 7.0.0
+      - uses: julia-actions/setup-julia@fa02766e078afaaf09b14210362cee14137e6a32 # 3.0.2
+      - uses: julia-actions/cache@a45e8fa8be21c18a06b7177052533149e61e9b38 # 3.1.0
+      - name: Install JuliaFormatter
+        run: |
+          julia -e 'using Pkg; Pkg.Registry.add("General"); Pkg.Registry.update(); Pkg.Apps.add(name="JuliaFormatter", version=v"2.10.1")'
+      - name: Check formatting
+        run: |
+          ${HOME}/.julia/bin/jlfmt --check --verbose .
+```
+
+If you want an action that makes comments on PRs with suggested formatting changes, you can use [the `julia-actions/julia-format` action](https://github.com/julia-actions/julia-format): please see that repository's documentations for instructions on how to set it up.
 
 Alternatively, if you have JuliaFormatter set up as part of your repository's [pre-commit checks](@ref pre-commit) checks, you can directly use pre-commit's CI tooling: