doc/faq: add a FAQ about versioning
doc/faq: add a FAQ about versioning

Fixes issue 5633.

R=golang-dev, r, tommi.virtanen, adg, nj
CC=golang-dev
https://codereview.appspot.com/14283044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang