Adding short paragraph about contributing to the README file.
Adding short paragraph about contributing to the README file.

gregmuellegger/django-sortedm2m,gradel/django-sortedm2m,gradel/django-sortedm2m,gradel/django-sortedm2m,fabrique/django-sortedm2m,gregmuellegger/django-sortedm2m,fabrique/django-sortedm2m,MathieuDuponchelle/django-sortedm2m,gregmuellegger/django-sortedm2m,MathieuDuponchelle/django-sortedm2m,fabrique/django-sortedm2m