doc: clarify what to do with Go 1.4 when installing from source

You have to actually run make.bash (or make.bat).

Update #18771.

Change-Id: Ie6672a4e4abde0150c1ae57cabb1222de2c78716
Reviewed-on: https://go-review.googlesource.com/35632Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install-source.html

@@ -147,6 +147,9 @@ either the git branch <code>release-branch.go1.4</code> or
 which contains the Go 1.4 source code plus accumulated fixes
 to keep the tools running on newer operating systems.
 (Go 1.4 was the last distribution in which the tool chain was written in C.)
+After unpacking the Go 1.4 source, <code>cd</code> to
+the <code>src</code> subdirectory and run <code>make.bash</code> (or,
+on Windows, <code>make.bat</code>).
 </p>
 
 <p>