doc: Add MinGW instructions to tutorial

Author: brson

doc/tutorial.md

@@ -71,6 +71,11 @@ and add `C:\Program Files\Rust\bin` to your `PATH` environment variable.
 You should then be able to run the rust compiler as `rustc` directly
 from the command line.
 
+On Windows Rust additionally requires [MinGW][mingw] (version
+20110802 is known to work).  It is currently recommended that windows
+development be done under the MinGW shell to ensure that the
+environment is configured correctly.
+
 We hope to be distributing binary packages for various other operating
 systems at some point in the future, but at the moment only windows
 binary installers are being made. Other operating systems must build
@@ -129,6 +134,7 @@ for more information on them.
 
 [installer]: http://dl.rust-lang.org/dist/rust-0.1-install.exe
 [tarball]: http://dl.rust-lang.org/dist/rust-0.1.tar.gz
+[mingw]: http://sourceforge.net/projects/mingw/files/Installer/mingw-get-inst/mingw-get-inst-20110802/mingw-get-inst-20110802.exe/download
 
 ## Compiling your first program