Added section on switching to customized version of Greenlight (#124)

Author: farhatahmad

_posts/2019-04-15-gl-customize.md

@@ -259,6 +259,14 @@ Once you've merged your code, you should look through the latest version of the
 
 # Customizing the Landing Page
 
+## Before you begin
+
+**IMPORTANT**
+
+If you installed using the `bbb-install.sh` script, then you must switch to the [Customize](gl-install.html#switching-from-install-to-customize) version of Greenlight first, before you proceed. 
+
+## Updating the code
+
 A common customization is to modify the default landing page. For a simple change, let's rename the welcome banner to say “Welcome to MyServer”.
 
 The welcome banner is generated by [index.html.erb](https://github.com/bigbluebutton/greenlight/blob/master/app/views/main/index.html.erb).  To customize this message, open `app/views/main/index.html.erb` in an editor.