Two small README changes

The first change fixes a small omission in the Controller spec example – response.code is a string, not a number, so it fails when you use eq(200).

The second change adds a link to the Capybara doc in the Request spec section, instead of only "hiding" in the rake customisation section.  I also added the explanation that the DSL is only available in spec/features by default, which is another thing I didn't find explained in the README; I was expecting the DSL to be available in the three directories outlined at the beginning of the section.  It may need a bit of work, but I tried to make it as clear as possible.