Add pipfile

[tombasche]

Pipenv is fast becoming the defacto way to do create a virtualenv for local development due to its simplicity.
To get started run:
pip install pipenv (either globally or with --user)
In the project directory run pipenv install.

[carltongibson]

...fast becoming the defacto way to...

Not sure it's quite over the line yet.

I'm a bit 👎 on this at the moment. Unless we're also going to delete the existing requirements files and use pipenv for everything, we're just adding a duplicate file to maintain. (And I'm not sure we're quite ready to make that change.)

@encode/django-rest-framework-core-team Opinions?

[carltongibson]

Closing as per encode/starlette#79

[xordoquy]

Seconded. Would make more sense for a complete website/API rather than only a library.