Adding similar build systems in the README

[codeanticode]

I think is a good idea to include a reference to similar cross-compilation systems based on MinGW, since it might be helpful for people who need something different from ossbuild, and also because they are a good reference to see how specific problems are solved (patches for misbehaving packages, compilation options, etc)

I found these two so far, which seem to be quite complete and up-to-date:

1. MinGW cross compiling environment:
   homepage: http://mingw-cross-env.nongnu.org/
   repo browser: http://hg.savannah.gnu.org/hgweb/mingw-cross-env
   mailing list: http://lists.nongnu.org/mailman/listinfo/mingw-cross-env-list

2. Fedora Cross Compiler Framework (generates 32 and 64 bits binaries)
   homepage: http://fedoraproject.org/wiki/MinGW/CrossCompilerFramework
   repo browser: http://svn.openftd.org/svn/fedora_cross/
   forum: http://sourceforge.net/mailarchive/forum.php?forum_name=mingw-w64-public