README: clarify license differences

README.md

@@ -26,4 +26,11 @@ but aims to make Thunderbird usable on devices with a small screens such as tabl
 
 ## License
 
-[Mozilla Public License 2.0 (MPL-2.0)](./LICENSE)
+[src](./src) is licensed under the
+[Mozilla Public License 2.0 (MPL-2.0)](./LICENSE).
+
+[extension](./extension) is licensed
+under the [General Public License v3.0 (GPL-3.0)](./extension/LICENSE)
+since the extension is based upon the 
+[work of Dillinger](https://addons.thunderbird.net/en-us/thunderbird/addon/toggle-message-pane/)
+which is licensed under the GPLv3.0 license.