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They were out of date and should refer to the source installation instructions.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

README

 This is the source code repository for the Go programming language.  
 
 For documentation about how to install and use Go,
-visit http://golang.org/ or load doc/install.html in your web browser.
+visit http://golang.org/ or load doc/install-source.html
+in your web browser.
 
 After installing Go, you can view a nicely formatted
-doc/install.html by running godoc --http=:6060
-and then visiting http://localhost:6060/doc/install.html.
+doc/install-source.html by running godoc --http=:6060
+and then visiting http://localhost:6060/doc/install/source.
 
 Unless otherwise noted, the Go source files are distributed
 under the BSD-style license found in the LICENSE file.