Improve Prompt Flow description in README.md

Add hint that AZURE_OPENAI_STREAM has to be set to False for prompt flow integration to work.

Motivation and Context

When you just set the prompt flow environt variables, it does not work and still uses OpenAI. You also have to set AZURE_OPENAI_STREAM to False which is not described in the documentation. This leads to confusion and already created an Issue #731 .

Description

I added a hint that to the description of the USE_PROMPTFLOW environment variable that AZURE_OPENAI_STREAM has to be set to False.

Contribution Checklist

• [x] I have built and tested the code locally and in a deployed app
• [x] For frontend changes, I have pulled the latest code from main, built the frontend, and committed all static files.
• [x] This is a change for all users of this app. No code or asset is specific to my use case or my organization.
• [x] I didn't break any existing functionality :smile:

Thanks @PhilippPeron for creating this PR