[release-branch.go1.2] misc/vim: describe how to get gofmt-on-save behaviour.

««« CL 31770043 / 286f0ddb799b
misc/vim: describe how to get gofmt-on-save behaviour.

Fixes #6826.

R=golang-dev, bradfitz, adg
CC=golang-dev
https://golang.org/cl/31770043
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/34960043