Create Workflow for Merge Notifications

Sent to Mattermost Channel for Certbot Team to check and be generally aware of more granular merge events.

.github/workflows/merged.yaml

@@ -0,0 +1,24 @@
+name: Merge Event
+
+on:
+  pull_request:
+    types:
+      - closed
+
+jobs:
+  if_merged:
+    if: github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+    steps:
+    - name: Create Mattermost Message
+      #https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable
+      env: 
+        NUMBER: ${{ github.event.number }}
+        PR_URL: https://github.com/${{ github.repository }}/pull/${{ github.event.number }}
+        REPO: ${{ github.repository }}
+        USER: ${{ github.actor }}
+      run: |
+        echo "{\"text\":\"[$REPO] | #[$NUMBER]($PR_URL) was merged into master by $USER.\"}" > mattermost.json
+    - uses: mattermost/action-mattermost-notify@master
+      env:
+        MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_MERGE_WEBHOOK }}