docs: add azure env vars (openai#814)

README.md

@@ -483,10 +483,10 @@ print(completion.model_dump_json(indent=2))
 
 In addition to the options provided in the base `OpenAI` client, the following options are provided:
 
-- `azure_endpoint`
+- `azure_endpoint` (or the `AZURE_OPENAI_ENDPOINT` environment variable)
 - `azure_deployment`
-- `api_version`
-- `azure_ad_token`
+- `api_version` (or the `OPENAI_API_VERSION` environment variable)
+- `azure_ad_token` (or the `AZURE_OPENAI_AD_TOKEN` environment variable)
 - `azure_ad_token_provider`
 
 An example of using the client with Azure Active Directory can be found [here](https://github.com/openai/openai-python/blob/main/examples/azure_ad.py).