Update README.md

Improved README.md to explain the steps needed to start the 'resque-web' server interface properly with the 'resque-retry' features. 

The problems as I identified them essentially were: 
A. The README proposed a rack configuration, but didn't include an example command for a user to start the server. Solution: Added a simple example on how to start the server with bundler by loading the proposed rack configuration. 
B. The README didn't include the alternate option of starting the resque-web interface using the built-in 'resque-web' command. Solution: Included a simple 3 step explanation on how to use the resque-web command by sending it a configuration file as a parameter
Thanks lantins. In this case, it's simply a configuration file inside a config folder, as per the examples in the resque documentation. The config directory is just there to follow convention, it's not really rails specific and works outside a rails project (which is my case). 

It's not a terrible idea to add Rails specific examples, such as how to embed the resque-web application with resque-retry tabs under a rails app, but I think what's here should suffice for now as it works for everyone that wants to simply run the server using the built-in command.