docs(gitlint): add documentation for Gitlint workflow

Closes: #70

README.md

@@ -12,6 +12,7 @@ various repositories of the Prosopograhy and Networks group at the
 
 * [add-to-project](docs/add-to-project.md)
 * [deploy-apis-instance.yml](docs/deploy-apis-instance.md)
+* [gitlint.yml](docs/gitlint.md)
 * [poetry-black](docs/poetry-black.md)
 
 * poetry-deptry.yml

docs/gitlint.md

@@ -0,0 +1,38 @@
+# Documentation for `gitlint.yml` workflow
+
+[gitlint.yml](https://github.com/acdh-oeaw/prosnet-workflows/.github/workflows/gitlint.yml) is a reusable GitHub workflow for linting Git commit messages with [Gitlint](https://jorisroovers.com/gitlint/latest/).
+
+
+## How to use
+
+Create a new workflow file in all repositories whose Git commit messages should get linted with Gitlint.
+
+The following example config runs the workflow on all pull requests which target the containing repository's default branch:
+
+```yml
+name: Run Gitlint
+
+on: 
+  pull_request:
+
+jobs:
+  gitlint:
+    uses: acdh-oeaw/prosnet-workflows/.github/workflows/gitlint.yml@RELEASE_VERSION
+    with:
+      basebranch: ${{ github.event.repository.default_branch }}
+```
+
+
+### Required settings
+
+* `RELEASE_VERSION` needs to be set to a valid prosnet-workflows [release tag](https://github.com/acdh-oeaw/prosnet-workflows/releases), branch name or SHA.
+
+### Optional settings
+
+The following optional variables can be added to the `with` section:
+
+* `arguments`:
+* `basebranch`:
+* `comment`:
+* `contrib`:
+* `ignore`: