link windows/cygwin build to the wiki docs

Suggested by temp01 in IRC.
http://irc.npmjs.org/node.js/2010-12-12#NODE-JS-2010-12-12T18-59-36-270Z
18:59:36 temp01: I feel like the build section on nodejs.org should link
there
19:11:18 isaacs: temp01: yeah, that'd be helpful

doc/index.html

@@ -103,7 +103,8 @@ net.createServer(function (socket) {
 
       <p>
         Node is tested on <b>Linux</b>, <b>Macintosh</b>, and
-        <b>Solaris</b>. It also runs on <b>Windows/Cygwin</b>,
+        <b>Solaris</b>. It also runs on
+        <b><a href="https://github.com/ry/node/wiki/Building-node.js-on-Cygwin-(Windows)">Windows/Cygwin</a></b>,
         <b>FreeBSD</b>, and <b>OpenBSD</b>. The build system requires Python
         2.4 or better.  V8, on which Node is built, supports only IA-32,
         x64, and ARM processors. V8 is included in the Node distribution.