add documentation on "base" plugins

[edaniszewski]

The notion of "base" plugins should be documented, where a "base" plugin is a layer of abstraction on top of the SDK which may provide a base for a given class of plugins. This base can provide common utility functions, protocol handling, etc, and may define easier ways for "subclassed" plugins define devices / handlers / etc.

A good example of this is with the work around generalizing the SNMP plugin, where the "base" provides handling for the SNMP protocol handling and device config management, and "subclassed" plugins only need to define devices for their MIB(s).