Contribution to the README for Capybara + database_cleaner

Hi!

Many devs use database_cleaner in conjunction with RSpec, Capybara and Selenium/Capybara-webkit.

I added one line in the readme to link to a post by Avdi Grimm where he shows a proper setup of database_cleaner to work with Capybara and includes a thorough explanation.

It seems to me that easying access to config instructions for such a popular setup will benefit both your gem and new developers.

Cheers!