doc: document go get -f flag in 1.4 release notes
doc: document go get -f flag in 1.4 release notes

LGTM=r, rsc
R=r, rsc, adg
CC=golang-codereviews
https://codereview.appspot.com/168890043

wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old