[README] Please include the following host dependencies

[chenxiaolong]

With the latest XBMC git, libsdl-image1.2-dev and liblzo2-dev is required to be installed on the host when cross compiling for the Raspberry Pi. It's specific required for TexturePacker.

Could you add those packages to the README file? Thanks!

[brooc]

Also swig and buildpoint needs to be added