Chat notifs: add licensing info

README.md

@@ -188,6 +188,8 @@ Customization details can be found in the [Customization doc](docs/customization
 
 Soapbox FE is based on [Gab Social](https://code.gab.com/gab/social/gab-social)'s frontend which is in turn based on [Mastodon](https://github.com/tootsuite/mastodon/)'s frontend.
 
+- `static/sounds/chat.mp3` and `static/sounds/chat.oga` are from [notificationsounds.com](https://notificationsounds.com/notification-sounds/intuition-561) licensed under CC BY 4.0.
+
 Soapbox FE is free software: you can redistribute it and/or modify
 it under the terms of the GNU Affero General Public License as published by
 the Free Software Foundation, either version 3 of the License, or