README install options are too complicated

There should be exactly two blessed ways to run the koans. Using lein 2 is a good idea to get people a good REPL and have them ready to actually do things when they're done w/ the koans. But there's also a lot of value in the easy zipfile-based approach where you just download, unzip, and go.

Calling those 2 paths out specifically would probably help, so people can follow one thread rather than searching for their option in every section.