Clarify Source Available terms for original project files

docs/README.en.md

@@ -85,3 +85,10 @@ The host changes; path, query, and hash stay the same.
 ## Diagnostics
 
 Use the diagnostics log in the options page when a rule does not switch as expected. You can also inspect the extension service worker from `chrome://extensions` and look for `[Auto Mirror Switcher]` logs.
+
+## License
+
+Original project files are licensed under the **[Source Available license](../LICENSE)**.
+Viewing the source, building an unmodified version and using the extension as intended are permitted.
+Modifications, forks, derivatives and redistribution require written permission from DepressedCat,
+subject to the exceptions in the license. Third-party components keep their own licenses.