Clarify

@cjolif and @pruzand, I wrote down what we discussed at the meeting but please take a look anyway.   The first commit adds the missing information and the second just rearranges everything into better sections.

Some points:

* I really fleshed out the section on testing.
* One thing we forgot to talk about was LESS... I know we decided to use Less rather than Stylus but I'm not sure if we also allow plain CSS or not.
* In the first section about end-user requirements, we should probably list which browsers we support.  Anything else?
Actually we're using the Intern Promised Based Webdriver API that wraps wd.js: https://github.com/theintern/intern/blob/1.7/lib/wd.js

It disappeared from the trunk, so hopefully it will disappear from Intern 2.0.