Add missing license statement

services/notifications/README.md

@@ -12,3 +12,9 @@ For notification expiry to work, a TTL index on `notifications.expires` must be
 ```javascript
 db.notifications.createIndex({expires: 1}, {expireAfterSeconds: 10})
 ```
+
+License
+=======
+The code in this repository is released under the GNU AFFERO GENERAL PUBLIC LICENSE, version 3.
+
+Copyright (c) Overleaf, 2016–2019.