Update readme to note how to disable Customizer

readme.md

@@ -16,6 +16,20 @@ The plugin ships with a default template that looks nice and clean and we tried
 
 You can tweak small pieces of the template or the entire thing depending on your needs.
 
+### AMP Customizer
+
+The plugin ships with its own Customizer that you can use to tweak various parts of the default template like colors.
+
+#### Disabling the AMP Customizer
+
+If you're using a completely custom template, you may want to disable the AMP Customizer:
+
+```
+add_filter( 'amp_customizer_is_enabled', '__return_false' );
+``` 
+
+Note that this needs to be called before the `after_setup_theme` hook to work.
+
 ### Where Do I Put My Code?
 
 The code snippets below and any other code-level customizations should happen in one of the following locations.