Add documentation discussing hierarchical interfaces

[mkorbel1]

Is your feature request related to a problem? Please describe. ROHD interfaces already support hierarchical interfaces, but there is no documentation or examples covering how to do it.

Describe the solution you'd like Add a section in the README that discusses hierarchical interfaces. Perhaps some examples and unit tests are valuable as well.

[mkorbel1]

There's an opportunity to provide some extra automation for sub-interface connections as well.