doc: document go1.4
doc: document go1.4

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/189810043

mapix/go,mapix/go,gomini/go-mips32,mapix/go,gomini/go-mips32,mapix/go,mapix/go,gomini/go-mips32,gomini/go-mips32,gomini/go-mips32,mapix/go,gomini/go-mips32,gomini/go-mips32,gomini/go-mips32,mapix/go,mapix/go