Add GitHub action for security check

.github/workflows/security.yml

@@ -0,0 +1,18 @@
+name: Security check with Brakeman
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+
+    - name: Setup Ruby and install gems
+      uses: ruby/setup-ruby@v1
+      with:
+        bundler-cache: true
+
+    - name: Run Brakeman
+      run: bundle exec brakeman -z