Adds a first cut for some documentation on how to write an API plugin
for V2.1. Note that this is not meant to have exhaustive coverage, and I
will add more detail in future commits. I do welcome suggestions of
other areas which should be included in future doc patches.
Partially implements blueprint v2-on-v3-api
Change-Id: Ibe5a471af357ce21253efeb8beb9ade1c16396f5
