Add compressed-size action (#420)

Author: kurkle

.github/workflows/compressed-size.yml

@@ -0,0 +1,14 @@
+name: Compressed Size
+
+on: [pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: preactjs/compressed-size-action@v2
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"