Added section to README for Zeus support

This gives instructions on how to implement Zeus support for RubyTest,
so you can utilize the preloaded rails environment within Sublime Text
2. Zeus is a replacement for spork. It allows you to preload a rails
environment which lets tests run instantly. It does more than spork did,
because instead of just allowing the preloading of the rails environment
for RSpec, it also preloads it for Cucumber, unit tests, running a
console, using script runner, using rake, and more.