Fix README.markdown instructions for reloading.

Original instructions didn't seem to work on Rails 3.2.x. Followed instructions
on wiki
(https://github.com/intridea/grape/wiki/Reload-Grape-API-on-every-request-in-Rails)
and it works without issue. In the original instructions, I could only get grape
to reload my endpoint once.

Using this submitted approach looks also looks like a standard way to hook
reloading into any inbound Rails request.