ci: add slack release notification

Author: cogwirrel

.github/workflows/on-release.yml

@@ -0,0 +1,24 @@
+name: Post release announcements
+
+on:
+  release:
+    types:
+      - published
+
+jobs:
+  slack-notification:
+    name: Share recent changes with a channel
+    runs-on: ubuntu-latest
+    steps:
+      - name: Send release notification
+        uses: slackapi/slack-github-action@v2.1.1
+        with:
+          webhook: ${{ secrets.SLACK_WEBHOOK_RELEASE_URL }}
+          webhook-type: webhook-trigger
+          payload: |
+            {
+              "name": ${{ toJSON(github.event.release.name) }},
+              "version": "${{ github.event.release.tag_name }}",
+              "body": ${{ toJSON(github.event.release.body) }},
+              "url": "${{ github.event.release.html_url }}"
+            }