Update README.md with build instructions.

These instructions were copied from teh Gumstix repo. Any mistakes are
mine.

https://github.com/gumstix/Gumstix-YoctoProject-Repo

Well, except they are confused by what "Yocto Project" and openEmbedded are :)

Signed-off-by: Philip Balister <philip@balister.org>