refactor(readme): add license section

Author: Akinator31

README.md

@@ -36,4 +36,8 @@ Project documentation is currently being written.
 
 ## Support
 
-If you need help, join the [Discord server][discord-invite] and ask for assistance in the #support channel.
+If you need help, join the [Discord server][discord-invite] and ask for assistance in the #support channel.
+
+## License
+
+This project is licensed under the [AGPLv3 License](https://github.com/Rustmail/rustmail/blob/main/LICENSE).