(doc) Split up README_DEVELOPER

The README_DEVELOPER document was getting too large and unwieldy. There was probably a lot of useful information in there, but it wasn't accessible enough for it to get the attention that it should. This splits it up into a `docs` directory where we can have much more focused information about the various aspects.

This was prompted by a request for some information on how to profile puppet. So there is also a new profiling document added.