docs: update README with usage and validation rules (#7)

Reviewed-on: #7
Co-authored-by: Jan Klattenhoff <jan@kjan.email>
Co-committed-by: Jan Klattenhoff <jan@kjan.email>

README.md

@@ -1,3 +1,41 @@
 # pull-request-lint
 
-Lints the title of a pull request
+A GitHub/Gitea Action that lints pull request titles to ensure they follow the [Conventional Commits](https://www.conventionalcommits.org/) format.
+
+## Usage
+
+Add the following to your GitHub/Gitea workflow:
+
+```yaml
+name: Pull Request Lint
+
+on:
+  pull_request:
+    types: [opened, edited, reopened, synchronize]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install go
+        uses: actions/setup-go@v5
+        with:
+          go-version: 1.24.2
+      - uses: https://git.kjan.de/actions/pull-request-lint@release
+```
+
+## Validation Rules
+
+The action enforces the following Conventional Commits rules for PR titles:
+
+- Format must be: `type(scope)!: description` (scope and breaking change marker `!` are optional)
+- Type and scope must be lowercase
+- Description must start with lowercase
+- Breaking change indicator and footer are mutually exclusive
+- Body must be separated from description by a blank line
+- Footer tokens must use hyphens instead of spaces (except for "BREAKING CHANGE")
+
+## License
+
+See [LICENSE](LICENSE) file for details.