Add note about Windows to Readme

README.md

@@ -37,6 +37,7 @@ Getting Started with Slate
 
 You're going to need:
 
+ - **Linux or OS X** — Windows may work, but is unsupported.
  - **Ruby, version 1.9.3 or newer**
  - **Bundler** — If Ruby is already installed, but the `bundle` command doesn't work, just run `gem install bundler` in a terminal.