Document the intended means of extending twistedchecker with new checks

[Jean-Paul Calderone]

in its current state, twistedchecker is a good start of a tool to check for compliance with Twisted's particular coding standard. It doesn't yet encode all of the Twisted coding standard, though, and the Twisted coding standard changes from time to time.

It would be great if there were a document explaining the basics of the way twistedchecker works from a somewhat high level and gave step by step directions for adding new checks (including which part of the overall system different kinds of checks best fit into, how to unit test checks, etc).