Update README.md

Removed warning about initializers and wrapped example initializer code in config.to_prepare blocks.

from http://guides.rubyonrails.org/configuring.html#initialization-events

> to_prepare: Run after the initializers are run for all Railties (including the application itself), but before eager loading and the middleware stack is built. More importantly, will run upon every request in development, but only once (during boot-up) in production and test.