[docs] Add rosetta instructions to docs

Summary:
Followup from https://linear.app/comm/issue/ENG-4374/problems-with-dev-environment-setup#comment-e374da31
Adding instructions on how to install roesetta for Apple silicon Macs

Test Plan: N/A

Reviewers: jon, ashoat, O6 Docs

Reviewed By: ashoat, O6 Docs

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8515

docs/nix_dev_env.md

@@ -27,6 +27,12 @@ Some workflows require additional steps after the Nix installation. These steps
 
 To set up a dev environment using Nix, you will need a macOS or Linux machine.
 
+If you are using a Mac computer with Apple silicon, you will need to install Rosetta 2. You can do this by running:
+
+```
+softwareupdate --install-rosetta
+```
+
 # Prerequisites
 
 ## Xcode