Add detailed OSX setup instructions to README.
Add detailed OSX setup instructions to README.

Fixes #1.

thesam/rOSt,pczarn/rustboot,tempbottle/rustboot,Arcterus/kRnel,alexchandel/rustboot,jvns/puddle,perlun/rustboot,smkell/rustboot,charliesome/rustboot,ryanra/RustOS,YangFan789/fanmi-os,thesam/rOSt,tempbottle/rustboot,RustOS-Fork-Holding-Ground/RustOS,jvns/puddle,wbthomason/ironkernel,smkell/rustboot,charliesome/rustboot