[Nix Docs] Mention homebrew packages

Summary:
Some tools don't exist in nixpkgs, mention them under the homebrew
section.

https://phab.comm.dev/D4978#149828

Test Plan: N/A

Reviewers: ashoat, abosh

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D5157