Documentation: driver design

[JohnStrunk]

Describe the feature you'd like to have.
The design & modularity of the codebase should be documented.

What is the value to the end user? (why is it a priority?)
Proper documentation will permit a developer to make maximum re-use of existing code for additional driver types while also using robust, approved APIs so as to keep modularity and driver types isolated from one another.

How will we know we have a good solution? (acceptance criteria)

• The driver-internal APIs should be fully documented (godoc)
• There should be a single document that provides an overview of these internal interfaces and how they should be used. Providing examples of how the modularity maximizes re-use between GD2 and Heketi glusterfs drivers is highly desirable.

Additional context
This item will need to wait for refactoring for Heketi to be completed.