Added GitHub Actions workflow.

.github/workflows/rubocop.yml

@@ -0,0 +1,18 @@
+name: CI
+
+on: [ push, pull_request ]
+
+jobs:
+  # rubocop linting
+  rubocop:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 3.0
+      - name: Install dependencies
+        run: bundle install --jobs 4 --retry 3
+      - name: Run rubocop
+        run: bundle exec rubocop --parallel