New Use_case is introduced.

I've also removed user setting of the return value, 
instead of ``return {'social_user': social_user, 'user': user}``
I used ``return {'social_user': social_user}``
Some indentations for cosmetics.
Merge pull request #637 from serdardalgic/use_case_doc_for_combining_two_functions_in_the_pipeline

Use case doc for combining two functions in the pipeline