Add GitHub Action to compress images automatically

.github/workflows/compress.yml

@@ -0,0 +1,18 @@
+name: Compress
+
+on:
+  pull_request:
+    paths:
+      - "src/**.png"
+
+jobs:
+  compress:
+    name: Images
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Compress Images
+        uses: calibreapp/image-actions@master
+        with:
+          githubToken: ${{ secrets.GITHUB_TOKEN }}