Update mkdocs version to 0.12.2

Fix markdown link references in release notes

The following items had a wrong href value:

- Dont require pk strictly for related fields. (#2745, #2754)
- Restrict integer field to integers and strings. (#2835, #2836)

Add Classy Django REST Framework(cdrf.co) to Third Party Resources

The old version of mkdocs required 'Markdown>=2.3.1,<2.5' and
requirements-optionals.txt lists markdown==2.5.2 causing mkdocs
to fail.

Support basic authentication with custom user models that change username field

On Tom's suggestion, improve coding style by using a single-line call to getattr() with a default value instead of a multi-line if/else clause.

Fix flake8 error

Support User model in Django 1.4 that has not a USERNAME_FIELD attribute.

Import get_user_model from compat module to be compatible with older django versions (e.g. 1.4).

Compatibility code for getting user model

Support basic authentication with custom user models with a username field that is not named 'username'.

Allow unexpected values for ChoiceField/MultipleChoiceField representations

Release 3.1.2

Allow DjangoObjectPermissions to use views that define get_queryset

minor grammar fix

Adding django-versatileimagefield to DRF 3rd party docs

Closes WGBH/django-versatileimagefield#12

Added link to djangorestframework-word-filter package

update info about wq.db.rest router

Separate LICENSE and README to two files.

Document correct method usage for get_paginated_response

Delete imported but not used method

Delete imported but not used method

 In 'Generating Tokens' section, the example code imported get_user_model but not used.

Fixed description for get_paginated_response method in generic-views and example using it with @list_route decorator in viewsets.

Fix Invalid keyword model with generic views