[release-branch.go1.4] [release-branch.go1.4] doc: update contribution guidelines

LGTM=minux, adg, rsc
R=rsc, r, dsymonds, minux, bradfitz, adg, dave, iant
CC=golang-codereviews
https://golang.org/cl/185190043
[release-branch.go1.4] doc: scrub references to code.google.com

These are the references that affect current Go users.
I left intact references in older release notes;
we can figure out what to do with them later.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/186140043

Conflicts:
	doc/go1.4.html

Change-Id: I1032686f2b3ac6dacaf8f114b8c35cdf221330ca
[release-branch.go1.4] api: create go1.4.txt

I read through and vetted these but others should look too.

LGTM=bradfitz, adg
R=r, minux, bradfitz, adg
CC=adg, golang-codereviews, gri, iant
https://golang.org/cl/182560043