Add set up of cross compile environment to README.

[ghostis]

For example, Ubuntu/Debian:

sudo apt-get -y install libtool m4 automake mingw-w64 autoconf

[pavangayakwad]

Seriously, all these are missing, i had to spend unnecessary time to explore one-by-one as and when the errors pop up. I always notice, most of the open source projects have worst setup documentation and ruins all the fun and high regard about the software when someone starting a fresh.