Fix README - HTTP status code is string [ci skip]

response.code is a string, so .eq(200) fails with:
     Failure/Error: expect(response.code).to eq(200)
       expected: 200
            got: "200"
Prefer response.status over response.code (which returns a string)
Add a bit of info about Capybara::DSL to README

I personally found it a bit confusing to figure out how to use the
Capybara DSL until I read the Capybara docs at the bottom of the
README.  It wasn't immediately obvious that this doc even /existed/
when I first read the README as I wasn't interested in customising
rake tasks.  So I have made another link to the doc in the request
spec section and explained that by default the DSL is only available
in spec/features.
Merge branch 'CorgiDude-patch-1'

[Closes #735]