Add docs and update templates for Django-React integration

[jeancochrane]

Overview

This PR adds documentation and updates the new-django-app template to recommend a pattern for using React components in a Django app.

Connects #66.

Testing Instructions

• View the rendered markdown, confirm it looks good and links resolve properly
• Change to the docker/templates directory and create a new app with docker-compose run --rm cookiecutter -f new-django-app
• Accept all default Cookiecutter options
• Change to the my-new-app repo and build container images with docker-compose build
• Follow the instructions in the Usage section of the new docs and use the example code to test the hybrid integration
• Visit http://localhost:8000 and confirm you see the text Welcome, foobar! below
• Once you're done testing, remember to clean up the testing app:
  • docker-compose down --volumes
  • cd ../ && rm -Rf my-new-app
  • docker rmi my-new-app