Add a file to link up extensions, with a reference in the readme

Author: florentdestremau

EXTENSIONS.md

@@ -0,0 +1,4 @@
+# [Dark and light mode theme switch](https://github.com/florentdestremau/omakub-darkmode-switch)
+
+Select a dark and light theme of your choice to be applied automatically when switching the gnome light/dark mode.
+

README.md

@@ -11,3 +11,9 @@ Please help us improve Omakub's documentation on the [basecamp/omakub-site repos
 ## License
 
 Omakub is released under the [MIT License](https://opensource.org/licenses/MIT).
+
+## Extras
+
+While omakub is purposed to be an opiniated take, the open source community offers alternative customization, add-ons, extras, that you can use to adjust, replace or enrich your experience.
+
+[⇒ Browse the omakub extensions.](EXTENSIONS.md)