Clarified build instructions and added links for downloading rtl-sdr tools.
Clarified build instructions and added links for downloading rtl-sdr tools.

nbred/rtlamr,loverso/rtlamr-old,nbred/rtlamr,bemasher/rtlamr,jomolinare/rtlamr,ejschaefer/rtlamr,curtiszimmerman/rtlamr,bemasher/rtlamr