Documenting how to write a new adapter

[karlcow]

When importing bugs from another bug tracker into Webcompat, We need to convert it. We call this interface an adapter.

We need a documentation explaining on how to contribute a new adapter to the project.

[miketaylr]

Agreed. I'd like for us to take a stab at writing the "easy" ones, like Chromium, WebKit, Bugzilla adapters, but the world is bigger than that. :)