Rakefile: added comments to build mingw32 binary for ruby-2.0.0
Rakefile: added comments to build mingw32 binary for ruby-2.0.0

nurse/msgpack-ruby,nurse/msgpack-ruby,kou/msgpack-ruby,larskanis/msgpack-ruby,cosmo0920/msgpack-ruby,ojundt/msgpack-ruby,nurse/msgpack-ruby,larskanis/msgpack-ruby,cosmo0920/msgpack-ruby,okkez/msgpack-ruby,kou/msgpack-ruby,kou/msgpack-ruby,kou/msgpack-ruby,okkez/msgpack-ruby,larskanis/msgpack-ruby,msgpack/msgpack-ruby,msgpack/msgpack-ruby,nurse/msgpack-ruby,ojundt/msgpack-ruby,okkez/msgpack-ruby,ojundt/msgpack-ruby,ojundt/msgpack-ruby,cosmo0920/msgpack-ruby,msgpack/msgpack-ruby,cosmo0920/msgpack-ruby,okkez/msgpack-ruby,larskanis/msgpack-ruby