doc: explain why cmd/go uses https for repositories and how to work around it
Fixes issue 3418.

R=golang-dev, r
CC=adg, golang-dev
https://codereview.appspot.com/7712045

Committer: Rob Pike <r@golang.org>
A+C: Herbert Georg Fischer (individual CLA)

Generated by addca.

R=gobot
CC=golang-dev
https://codereview.appspot.com/7861044