Create Weekly Notification Message for Certbot Team

Composes Mattermost message to team channel

.github/workflows/notify_weekly.yaml

@@ -0,0 +1,25 @@
+name: Weekly Github Update
+
+on:
+  schedule:
+    # Every week on Thursday @ 1:00
+    - cron: "0 1 * * 4"
+jobs:
+  send-mattermost-message:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Create Mattermost Message
+      run: |
+        DATE=$(date  --date="7 days ago" +"%Y-%m-%d")
+        MERGED_URL="https://github.com/pulls?q=merged%3A%3E${DATE}+org%3Acertbot+"
+        UPDATED_URL="https://github.com/pulls?q=updated%3A%3E${DATE}+org%3Acertbot+"
+        echo "{\"text\":\"## Updates Across Certbot Repos\n\n
+        - Certbot team members SHOULD look at: [link]($MERGED_URL)\n\n
+        - Certbot team members MAY also want to look at: [link]($UPDATED_URL)\n\n
+        - Want to Discuss something today? Place it [here](https://docs.google.com/document/d/17YMUbtC1yg6MfiTMwT8zVm9LmO-cuGVBom0qFn8XJBM/edit?usp=sharing) and we can meet today on Zoom.\n\n 
+        - The key words SHOULD and MAY in this message are to be interpreted as described in [RFC 8147](https://www.rfc-editor.org/rfc/rfc8174). \"
+        }" > mattermost.json
+    - uses: mattermost/action-mattermost-notify@master
+      env:
+        MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_WEBHOOK_URL }}