Update readme

Before: https://github.com/leapmotion/leapjs
After: https://github.com/leapmotion/leapjs/tree/readme

This can be merged.  Next step: rewrite https://developer.leapmotion.com/leapjs/getting-started to place the node items correctly.

CC @supernancychen @naorbrown @dpetersonLeap in case any of you guys are interested in reviewing these changes.