doc/install.html: document use of libc6-dev-i386

Fixes #1689.

R=rsc
CC=golang-dev
https://golang.org/cl/4404043

doc/install.html

@@ -91,7 +91,9 @@ installed as part of
 <a href="http://developer.apple.com/TOOLS/Xcode/">Xcode</a>. 
 </p>
 
-<p>On Ubuntu/Debian, use <code>sudo apt-get install bison ed gawk gcc libc6-dev make</code>.
+<p>On Ubuntu/Debian, use <code>sudo apt-get install bison ed gawk gcc libc6-dev
+make</code>. If you want to build 32-bit binaries on a 64-bit system you'll
+also need the <code>libc6-dev-i386</code> package.
 </p>
 
 <h2 id="mercurial">Install Mercurial, if needed</h2>