chore(docs): update README to include links to docs on Webhooks

Author: dtmeadows

README.md

@@ -116,6 +116,8 @@ Note that you can also pass a raw `IO` descriptor, but this disables retries, as
 
 Verifying webhook signatures is _optional but encouraged_.
 
+For more information about webhooks, see [the API docs](https://platform.openai.com/docs/guides/webhooks).
+
 ### Parsing webhook payloads
 
 For most use cases, you will likely want to verify the webhook and parse the payload at the same time. To achieve this, we provide the method `client.webhooks.unwrap`, which parses a webhook request and verifies that it was sent by OpenAI. This method will raise an error if the signature is invalid.