extensions: and ui note and fix index section

Author: Pablo Panero

docs/extensions/custom.md

@@ -59,3 +59,6 @@ It's all set, run your instance with the cli and you will have your new features
 ``` console
 $ invenio-cli run
 ```
+
+!!! note "UI related extensions"
+    if your extension adds scss and/or javascript, you will need to update your final static files before running your instance! You can do so with the CLI: `invenio-cli update [--no-install-js]`

docs/extensions/index.md

@@ -1,8 +1,12 @@
 # What can be added?
 
-InvenioRDM supports a great variety of extensions that can help you adapt it to your infrastructure. The currently supported extensions are listed below. If you need even more, you can develop your own! Check how to do so [here](custom.md).
+InvenioRDM supports a great variety of extensions that can help you adapt it to your infrastructure. The currently supported extensions are listed below.
 
-## Supported extensions
+## Develop your own extension
+
+If you want to add custom functionality to your RDM instance, you need to develop your own module. We have the docs and cookiecutter template to get you started in no time.
+
+[> Develop an extension](custom.md)
 
 ### Storage