Update README.md based on Connor's flow

[pamelafox]

Purpose

This PR makes some README adjustments based off debugging session with Connor.

Specifically, Connor was trying to make it work without running azd up, so I'm adding more detail to that bullet point to try to clarify its necessity.

I also changed az account show to az ad signed-in-user show as that seems to be the right way to get a principal ID that works in multi-tenant situations.

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[ ] Other... Please describe: