Add publish workflow

.github/workflows/publish.yml

@@ -0,0 +1,19 @@
+name: Publish package to pub.dev
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Publish
+        uses: sakebook/[email protected]
+        with:
+          credential: ${{ secrets.CREDENTIALS_JSON }}
+          skip_test: true
+          flutter_package: false