Add testing information to the README

[penguinpowernz]

This helped me integrate Authie into an existing app that used Devise/Warden.

I think the modules from the README could actually be included in the codebase as:

• lib/authie/test/devise_helper.rb
• lib/authie/test/warden_helper.rb

Then people should be able to include it like Devise does in their spec_helper file:

require `authie/test/devise_helper`

RSpec.configure do |config|
  config.include Devise::Test::ControllerHelpers, :type => :controller
  config.include Authie::Test::DeviseHelpers, :type => :controller # this must come after the devise one
end

We could even add the setup_session function from the dummy app in this repo to lib/authie/test/controller_helpers.rb to make it easier to test on normal Authie apps.

Let me know if this is worth me pursuing this.