docs(actors): builtin-actors development docs

[rvagg]

Ref: https://github.com/filecoin-project/builtin-actors/issues/1567

@rjan90 @BigLep I started banging this out as I was closing out 0081 and preparing a fresh branch based on these instructions for @snissn to get him started on MCOPY. I'll finish off the "Builtin-actors-only changes" section tomorrow, along with the MCOPY starter branch, unless something else more urgent comes up. The other sections can remain TODO until later, although the state migration one probably isn't too hard, now that I've done it for 0081; but it can be done separately.