Adding some documentation on hacking on yapf
Adding some documentation on hacking on yapf

Currently info on releases

bygloam/yapf,lucius-feng/yapf,abduld/yapf,ambv/yapf,jamesblunt/yapf,google/yapf,sbc100/yapf,elviswf/yapf,dmoliveira/yapf,sbc100/yapf,nikolamilosevic86/yapf,google/yapf,thurday/yapf,hayd/yapf,wklken/yapf