Updated the README to point people at Cholla

I'm not going to be doing any more work on Sickle as I find I generally prefer Cactus to Hammer. But I've created a Cactus skeleton that's very similar to Sickle where I will continue to add new stuff: https://github.com/jimray/cholla
Merge branch 'master' of https://github.com/jimray/sickle

Conflicts:
	README.md