Add a GitHub action for linting.

.github/workflows/lint.yml

@@ -0,0 +1,15 @@
+name: Linter
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.4
+        bundler-cache: true
+    - name: Rubocop
+      run: bundle exec rubocop