[docs] add instructions for creating a new user

Summary: We instruct the developer to create a new user through the iOS simulator.

Test Plan: This is how I've created new users locally in the past

Reviewers: ashoat, atul, yayabosh

Reviewed By: ashoat, yayabosh

Subscribers: palys-swm, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4419