added note about the GOARM env variable

R=rsc, r
CC=golang-dev
https://golang.org/cl/198074

doc/install.html

@@ -86,6 +86,17 @@ plus one optional variable:</p>
 	After installing, you will want to arrange to add this
 	directory to your <code>$PATH</code>, so you can use the tools.
 </dd>
+
+<dt>
+<code>$GOARM</code> (optional, arm, default=6)
+</dt>
+<dd>
+	The ARM architecture version the runtime libraries should target.
+	ARMv6 cores have more efficient synchronization primitives. Setting
+	<code>$GOARM</code> to 5 will compile the runtime libraries using
+	just SWP instructions that work on older architectures as well.
+	Running v6 code on an older core will cause an illegal instruction trap.
+</dd>
 </dl>
 
 <p>