[themes] Mention license files in README

runtime/themes/README.md

@@ -6,4 +6,12 @@ If you submit a theme, please include a comment at the top with your name and em
 # Author : Name <email@my.domain>
 ```
 
+If you are submitting a theme that is already published somewhere, then please
+add the corresponding license file for it under `licenses/` with the following
+name `theme_name.license` and add a comment at the top of the theme file:
+
+```toml
+# License: <License specification>
+```
+
 We have a preview page for themes on our [wiki](https://github.com/helix-editor/helix/wiki/Themes)!