Add initial nix documentation

Summary: Demonstrate how to install and setup nix for development.

Test Plan:
Follow steps in nix_dev_env.md to install nix

Optionally, install direnv if auto-instantiating shell is desired.

Reviewers: atul, palys-swm, karol-bisztyga, geekbrother, varun, ashoat

Reviewed By: atul, palys-swm, varun, ashoat

Subscribers: benschac, ashoat, palys-swm, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D2921