[release-branch.go1.4] doc: copy contribute.html and install-source.html from master

This incorporates the various git-related updates that have
happened since the Go 1.4 release. Since Go 1.4.1 will be issued
from Git, it is appropriate to replace the Mercurial instructions
with Git instructions.

Change-Id: Idec041002c7f325c4eee6f25c50423b088b11468
Reviewed-on: https://go-review.googlesource.com/2788


Reviewed-by: Andrew Gerrand <adg@golang.org>