ci: add a skeleton github actions workflow for downstream projects

See html5lib#141

Author: flavorjones

.github/workflows/downstream.yml

@@ -0,0 +1,21 @@
+name: downstream
+
+concurrency:
+  group: "${{github.workflow}}-${{github.ref}}"
+  cancel-in-progress: true
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - master
+  pull_request:
+    types: [opened, synchronize]
+    branches:
+      - '*'
+
+jobs:
+  skeleton:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo hello world