[release-branch.go1.4] doc: document Go 1.4.2

Change-Id: Ia87047cbc720fb03d2f67aec48abe18bce8dbf78
Reviewed-on: https://go-review.googlesource.com/5112
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/5113
Reviewed-by: Andrew Gerrand <adg@golang.org>