Add rubocop github action step

.github/workflows/rubocop_lint.yml

@@ -0,0 +1,27 @@
+name: Lint 💎
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby 2.6 is
+      uses: actions/setup-ruby@v1
+      with:
+        ruby-version: 2.6.x
+    - uses: actions/cache@v1
+      with:
+        path: vendor/bundle
+        key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-gems-
+    - name: Run Rubocop linter
+      run: |
+        gem install bundler
+        bundle config path vendor/bundle
+        bundle install --jobs 4 --retry 3
+        bundle exec rubocop