[release-branch.go1.4] doc: document Go 1.4.1

Change-Id: I4e9737497f4995657c46e52e0722d921499f8d17
Reviewed-on: https://go-review.googlesource.com/2854
Reviewed-by: Rob Pike <r@golang.org>
(cherry picked from commit 7785be8f2227df881e7ab3a4e81a2fc69a165cf8)
Reviewed-on: https://go-review.googlesource.com/2855