No cross-compiling instructions on INSTALL

Hello,

I'm just skimming through INSTALL, and I didn't find any mention to cross-compiling Git. I think it would be nice to add such instructions to INSTALL, or to separate INSTALL_CROSS.

The idea of cross-compiling is to compile Git targeting different architecture and/or OS than build machine (for example, compiling for ARM on x86(_64).

Note that for cross-compiling we need a cross-compiler targeting the desired target, and for binary compatibility with rest of target system, we need versions of compiler, libc, and binutils match the ones on the target.

Thanks.