Add documentation for adding a custom layout to an application

[jeff-phillips-18]

Describe the problem
Applications may want to use a layout other than one of the provided layouts. This can be done but there is no clear documentation on what an application would need to do in order to use their own layout.

Expected behavior
There should be some documentation on what the custom layout should provide in order to be consumed and how to specify the new custom layout.

/cc @kialam