Add a GitHub Actions workflow for PHPCS

Ensure that the code matches the PSR-12 coding standards by
automatically running PHPCS for each push and pull request.

Fixes #5

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+---
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  phpcs:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        php-version: ['7.4']
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Configure PHP ${{ matrix.php-version }}
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-version }}
+
+      - name: Install Composer dependencies
+        run: >
+          composer install --no-progress --no-suggest --prefer-dist \
+            --optimize-autoloader
+
+      - name: Run PHPCS
+        run: vendor/bin/phpcs -v