Add writeup about using with Spring to README

would you mind basically transplanting your blog post here? (Also perhaps rename the heading to 'Configuring Spring to work with simplecov').  I'm also curious about what the use-case would be for using spring with simplecov? 


>Create a config/spring.rb file and move your SimpleCov config into it from your spec_helper.rb file. For example:
>

```ruby
if ENV['RAILS_ENV'] == 'test'
  require 'simplecov'
  SimpleCov.start
end
```