[release-branch.go1.2] README: Fix installation instructions

««« CL 28500043 / 1140207a3395
README: Fix installation instructions
They were out of date and should refer to the source installation instructions.

Fixes #6783.

R=golang-dev, rsc, adg, dave
CC=golang-dev
https://golang.org/cl/28500043
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/34880045