Developer book

I think we should add a developer guide here. This would be a separate book, so there would be the User section (which I believe is what this currently is) and the Developer section.

We need this because developer documentation should be a first-class citizen and the quality of such a document should be important otherwise it will not be used.

For example I want to write a guide on how to write behat tests, standardize UI workflows (e.g. there should always be a success / error notification, delete buttons should always be here, etc.).

Doing this in markdown in the existing `/docs` repository which is of variable quality does not really appeal to me and browsing such documents on GITHub isn't the best experience - RST provides a much richer medium for technical documentation and readthedocs provides a great reading experience.
Indeed .. I will check to see what behat 3 does  by default.