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