docs: link to custom HTTP client instructions (#513)

Author: thinkingserious

README.md

@@ -123,6 +123,8 @@ message_sid = 'SMxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
 @client.http_client.adapter = :typhoeus
 ```
 
+To use a custom HTTP client with this helper library, please see the [Twilio documentation](https://www.twilio.com/docs/libraries/ruby/custom-http-clients).
+
 ### Handling Errors
 
 ```ruby