Install README.md, COPYRIGHT, LICENSE-MIT, LICENSE-APACHE

[brson]

These are currently included in the binary tarballs, but not actually installed. They need to be part of the installation to comply with the licenses. Probably put them in ${prefix}/share/doc/rust. Make sure the Mac .pkg and Windows .exe installers do too.

[brson]

For the windows installation we might also put that 'third-party' directory that deals with the mingw components into this doc directory.

[alexcrichton]

I believe we do this now, so closing.