Clarify that YAPF currently requires Python 2.7 and 3.x is planned.
Clarify that YAPF currently requires Python 2.7 and 3.x is planned.

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