Add matrix drone notifications

.drone.yml

@@ -8,3 +8,13 @@ steps:
       - opam pin add -n .
       - opam --yes depext cert_service
       - opam install -t -y cert_service
+
+  - name: notify
+    image: plugins/matrix
+    settings:
+      homeserver: https://data.coop
+      roomid: !AFqVDyVtyrkCETxSWt:data.coop
+      username:
+        from_secret: matrix_username
+      password:
+        from_secret: matrix_password