[doc] added errors section for 403

[shiv248]

Hello,

before this repo gets a flurry of people having 403 errors, I thought of updating the readme to include why that is the reason at least for this week, until the full rollout of Realtime API comes along.

resources: openai community - realtime Websocket error unexpected server response 403

openai community - my org has no access to the realtime api

https://github.com/openai/openai-realtime-console/issues/4

openai realtime API playground

can adjust pr if necessary.

[shiv248]

can ignore and close pr without merging if full rollout already happened.