Document polymorphic associations in README.md
Add tests for has_many polymorphic
Add tests for has_one polymorphic
Separate associations into multiple files
Add support to polymorphic models
Fix #0d31e72 regression

https://github.com/rails-api/active_model_serializers/commit/0d31e72d2211b6bf7f0b0420139c4b370d6e986e