Create Usage Plugin API doc

[reshmabidikar]

A usage plugin API doc needs to be created similar to the entitlement plugin doc. This should cover the use cases for usage plugin API.

[sbrossie]

Outline of the doc should be similar to the other ones we have:

Overview

Provide pointer to usage api and a description of how it works

Use Case

This is used to fetch usage data from a third party metering system.