Add matrix release alert script and gitlab job (#688)

* Add matrix release alert script and gitlab job

* Update .gitlab-ci.yml

Co-Authored-By: Kirill Pimenov <kirill@parity.io>

polkadot/.gitlab-ci.yml

@@ -111,8 +111,14 @@ check-line-width:
   interruptible:                   true
   allow_failure:                   true
 
-
-
+alert-pending-release:
+  stage:                           test
+  image:                           parity/tools:latest
+  only:
+    - tags
+    - /^v[0-9]+\.[0-9]+\.[0-9]+.*$/        # i.e. v1.0.1, v2.1.0rc1
+  script:
+    - ./scripts/gitlab/alert_pending_release.sh
 
 test-linux-stable:                 &test
   stage:                           test
@@ -365,4 +371,3 @@ deploy-polkasync-kusama:
         ${GITLAB_API}/projects/${GITHUB_API_PROJECT}/trigger/pipeline | jq .
 
 
-