Update README with Expensify engineer instructions

[NicolasBonet]

Added instructions for Expensify engineers regarding email configuration and CORS issues.

Explanation of Change

Points into the right direction after running the iOS/Android apps for expensify engineers pointing to a local VM regarding errors with using their email accounts to start using the app.

Fixed Issues

$#88164

[github-actions]

👋 Hi @NicolasBonet, thanks for your interest in contributing to Expensify!

This PR has been automatically closed because it doesn't appear to meet our contribution requirements:

• You are not a member of the Expensify GitHub organization
• No linked GitHub issue was found in the PR description where you are listed as an assignee
• No linked GitHub PR was found in the PR description where you are the author or a reviewer

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[grgia]

@NicolasBonet I can't add a review because this issue is locked. BUT

Rather than adding a note to the main doc, could we add this step in the correct place in the set up flows?

It's unclear where you saw this error in your flow, even from the linked issue