Add a bit of info about Capybara::DSL to README
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.

DarthSim/rspec-rails,ipmobiletech/rspec-rails,jpbamberg1993/rspec-rails,powershop/rspec-rails,jamelablack/rspec-rails,eliotsykes/rspec-rails,rspec/rspec-rails,gkunwar/rspec-rails,pjaspers/rspec-rails,sarahmei/rspec-rails,jamelablack/rspec-rails,ipmobiletech/rspec-rails,Kriechi/rspec-rails,ipmobiletech/rspec-rails,mkyaw/rspec-rails,AEgan/rspec-rails,powershop/rspec-rails,mcfiredrill/rspec-rails,jdax/rspec-rails,gkunwar/rspec-rails,maclover7/rspec-rails,gkunwar/rspec-rails,tjgrathwell/rspec-rails,rspec/rspec-rails,jamelablack/rspec-rails,unite-us/rspec-rails,jasnow/rspec-rails,tjgrathwell/rspec-rails,sarahmei/rspec-rails,pjaspers/rspec-rails,jasnow/rspec-rails,sopheak-se/rspec-rails,aceofspades/rspec-rails,pjaspers/rspec-rails,ResultadosDigitais/rspec-rails,KlotzAndrew/rspec-rails,Kriechi/rspec-rails,AEgan/rspec-rails,dcrec1/rspec-rails-1,DarthSim/rspec-rails,grantgeorge/rspec-rails,unite-us/rspec-rails,powershop/rspec-rails,KlotzAndrew/rspec-rails,AEgan/rspec-rails,aceofspades/rspec-rails,KlotzAndrew/rspec-rails,dcrec1/rspec-rails-1,eliotsykes/rspec-rails,dcrec1/rspec-rails-1,jdax/rspec-rails,Kriechi/rspec-rails,mcfiredrill/rspec-rails,jpbamberg1993/rspec-rails,grantgeorge/rspec-rails,ResultadosDigitais/rspec-rails,mkyaw/rspec-rails,maclover7/rspec-rails,sarahmei/rspec-rails,DarthSim/rspec-rails,aceofspades/rspec-rails,maclover7/rspec-rails,jdax/rspec-rails,jpbamberg1993/rspec-rails,sopheak-se/rspec-rails,sopheak-se/rspec-rails,mcfiredrill/rspec-rails,rspec/rspec-rails,eliotsykes/rspec-rails,unite-us/rspec-rails,mkyaw/rspec-rails,jasnow/rspec-rails,grantgeorge/rspec-rails,tjgrathwell/rspec-rails