Update README with Expensify engineer instructions

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

Author: NicolasBonet

README.md

@@ -55,6 +55,7 @@ For detailed setup instructions for each platform, see the following guides:
    Access to fetch at 'https://www.expensify.com/api/BeginSignIn' from origin 'http://localhost:8080' has been blocked by CORS policy
    ```
    You probably have a misconfigured `.env` file - remove it (`rm .env`) and try again.
+3. Expensify engineers having an error `The given email is not valid` must [follow this instructions](https://stackoverflowteams.com/c/expensify/questions/23537/23538#23538) to configure the local dev email account.
 
 **Note:** Expensify engineers that will be testing with the API in your local dev environment please refer to [these additional instructions](https://stackoverflow.com/c/expensify/questions/7699/7700).