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?
OK.

About has(), I could list it in the AMD section too, but it's weird to think of it as a plugin... it's mainly just used as a plain module.   I'm also unclear why you added has() to requirejs-dplugins, was it because we haven't created a core project yet?