Better documentation for building on Windows

[darkf]

I'd love to contribute, but I find that the docs on building (particularly for Windows) is lacking.

• It doesn't say whether to use MSYS or Cygwin
• Cygwin doesn't seem to be supported (is there any reason for this?)

[brson]

A lot has changed in 5 months. The docs could be better still, but the specific issue of whether to use msys or cygwin is addressed.

Current instructions for windows are here: https://github.com/mozilla/rust/wiki/Doc-getting-started