Create README.md
Create README.md

Document the current set of operators
walkingsmarts/ReactiveCocoa,calebd/ReactiveCocoa,calebd/ReactiveCocoa,neilpa/Rex,ikesyo/Rex,walkingsmarts/ReactiveCocoa,ohwutup/ReactiveCocoa,RACCommunity/Rex,RACCommunity/Rex,skyylex/Rex,avalanched/Rex,ohwutup/ReactiveCocoa,neilpa/Rex,ikesyo/Rex,bjarkehs/Rex,nanoxd/Rex,walkingsmarts/ReactiveCocoa,calebd/ReactiveCocoa,bjarkehs/Rex,ohwutup/ReactiveCocoa,nanoxd/Rex,RACCommunity/Rex