Add README hint for bitness under windows
Add README hint for bitness under windows

I am personally not sure if issues of this kind exist on
other platforms, but I did not encounter them.

Also see: https://github.com/rmagick/rmagick/issues/225
rmagick/rmagick,carsonreinke/rmagick,rmagick/rmagick,carsonreinke/rmagick,carsonreinke/rmagick,rmagick/rmagick