(Readme) Add detailed env-var instructions

[maxsu]

If applied, this patch with update the Readme to show users how to properly export the OPEN_API_KEY env var when running main-local.py.

This solves the cause of issue #15 - Confusion about env vars.

I spoke with author of #15. They had not exported the env var properly:

• User believed the error message was caused by not having enough VRAM to run OpenAI APIs in "offline mode"
• I explained that "offline mode" means that main-local.py runs a flask server locally, but still depends on external OpenAI APIs
• I instructed user on setting Env Vars
• We verified that main-local.py runs after setting the env var

No breaking changes

[mukulpatnaik]

Thanks for this, this was added in a previous refactor