Fail better. This changes mirrors `django.template.loader_tags.IncludeNode`. I didn't include it in the previous commit to show test consistency.
Tests for render_model with/only.

Using similar scenarios as Django uses to test `include` in `tests.regressiontests.templates.tests`.
WIP readme updates

- add render_model with/only documentation
- add a cheat sheet of all template tags
- add documentation of render_list and render_iter