Add integration workflow.

Author: nsaunders

.github/workflows/integration.yml

@@ -0,0 +1,19 @@
+name: Integration
+
+on:
+  push:
+    branches: [master]
+    paths-ignore: ['**.md']
+
+  halogen:
+    name: Halogen
+ 
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Notify of change
+        run: |
+          curl -X POST https://api.github.com/repos/nsaunders/purescript-tecton-halogen/dispatches \
+          -H 'Accept: application/vnd.github.everest-preview+json' \
+          -u ${{ secrets.GH_ACCESS_TOKEN }} \
+          --data '{"event_type": "dependency_changed", "client_payload": { "repository": "'"$GITHUB_REPOSITORY"'" }}'