Guide for authors interested in writing adapters and extensions

[maetl]

This would help fill some of the gaps in knowledge around extensions and take some of the pressure off the contributors guide (see: #43), in terms of helping people to ramp up as contributors to ROM.

A guide should address the following details:

• The range of different possibilities for extending ROM
• The kinds of things that we want to see built
• High level description of the dataset and repository interface; examples from existing adapters
• Provided API hooks for extensions to work with
• How to write code that plays nicely with ROM
• Naming things
• Required git repo layout, gem spec, module, and testing structure

[solnic]

:+1: this definitely deserves a separate sub-section in "guides"