Add example in documentation to run action only for changed files (#98)

README.md

@@ -66,6 +66,39 @@ jobs:
 
 **You can copy/paste the `.github/` folder (under `examples/`) to your project and that's all!**
 
+## Usage with only changed files
+
+It is also possible to run PHP-CS-Fixer just on your changed files. To achieve this, you can use the [`tj-actions/changed-files`](https://github.com/tj-actions/changed-files) action to retrieve the changed files and subsequently use the result to create extra arguments with `---path-mode=intersection`.
+
+```
+on: [push, pull_request]
+name: Main
+jobs:
+  php-cs-fixer:
+    name: PHP-CS-Fixer
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Get changed files
+        id: changed-files
+        uses: tj-actions/changed-files@v38
+
+      - name: Get extra arguments for PHP-CS-Fixer
+        id: phpcs-intersection
+        run: |
+          CHANGED_FILES=$(echo "${{ steps.changed-files.outputs.all_changed_and_modified_files }}" | tr ' ' '\n')
+          if ! echo "${CHANGED_FILES}" | grep -qE "^(\\.php-cs-fixer(\\.dist)?\\.php|composer\\.lock)$"; then EXTRA_ARGS=$(printf -- '--path-mode=intersection\n--\n%s' "${CHANGED_FILES}"); else EXTRA_ARGS=''; fi
+          echo "PHPCS_EXTRA_ARGS<<EOF" >> $GITHUB_ENV
+          echo "$EXTRA_ARGS" >> $GITHUB_ENV
+          echo "EOF" >> $GITHUB_ENV
+
+      - name: PHP-CS-Fixer
+        uses: docker://oskarstark/php-cs-fixer-ga
+        with:
+          args: --config=.php-cs-fixer.dist.php -v --dry-run --stop-on-violation --using-cache=no ${{ env.PHPCS_EXTRA_ARGS }}"
+```
+
 ## Docker
 
 A Docker image is built automatically and located here: