Wrote various parts of the notes for RPerf

* started the introduction, sketching the problems with the classical
  case and outlining two examples of derived moduli spaces
* a bit more about what a derived stack is
* explicit reference to HAG-II for what to prove
Wrote a very sketchy proof of derived stackiness

It needs to be improved, but it can serve as the "real" proof in this
text. The others can be merely sketches.
Two more incomplete sketches of proofs
Improved introduction up to now
Discussed RVect as motivation for DAG
Added references for the introduction

I think / hope the introduction is now finished.
Finished the proof of the stackiness of RPerf

At least, with one last caveat that should be (almost) trivial.