add sample

.github/workflows/notify.yaml

@@ -0,0 +1,16 @@
+name: notify
+
+on:
+  workflow_call:
+jobs:
+  gradle-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: mattermost/action-mattermost-notify@master
+        with:
+          MATTERMOST_WEBHOOK_URL: ${{ secrets.MM_WEBHOOK_URL }}
+          PAYLOAD: |
+            { 
+              "text": "${{ github.repository }} [Pipeline](${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_number }}) was canceled :man_facepalming:" 
+            }
+