Add serialization_scope to README

[rafaelcgo]

Just a small suggestion that might help others...

I was struggling with undefined method 'authenticate' for nil:NilClass for my requests using the serializer (Rspec).

Turns out all I needed was to add serialization_scope :view_context but I only discovered it when watching the amazing Railcast about the gem: http://railscasts.com/episodes/409-active-model-serializers?view=asciicast

I think you guys should add serialization_scope documentation to the readme, to make things easier. And refer to the Railscast as well..

[bf4]

@rafaelcgo in case you're interested, I'm documenting this in #1252

Thanks for the view_context idea

[rafaelcgo]

I'm glad I could help! 👏