misc/vim: describe how to get gofmt-on-save behaviour.
misc/vim: describe how to get gofmt-on-save behaviour.

Fixes issue 6826.

R=golang-dev, bradfitz, adg
CC=golang-dev
https://codereview.appspot.com/31770043

tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go