[RFC] Document Python API

[ryneeverett]

From #784:

Bugwarrior is likely being used as a library by some folks and semver communicates the API compatibility. In my opinion internal API compatibility is not a reasonable expectation for applications unless the API is documented or it is at least communicated by the maintainers that usage as a library is supported. @coddingtonbear does not agree with this. Perhaps we can agree that documenting the API or at least the fact that library usage is supported wouldn't do any harm.