text: Title: Add 'pip install -e .' in Makefile
  Body: Hello, I just discovered this repo and it looks great!
  I read the blog post: https://drivendata.github.io/cookiecutter-data-science/
  
  And I am wondering if some of the mentioned commands should be in the Makefile, like:
  `pip install -e .`
  `pip freeze > requirements.txt`
  
  