Add Intercom::GatewayTimeoutError to README (#574)

Co-authored-by: Sean Healy <sean.healy@intercom.io>
Co-authored-by: Johnny Nader <30523158+johnnynader@users.noreply.github.com>

Author: 3 people

README.md

@@ -696,6 +696,7 @@ Intercom::RateLimitExceeded
 Intercom::AttributeNotSetError # Raised when you try to call a getter that does not exist on an object
 Intercom::MultipleMatchingUsersError
 Intercom::HttpError # Raised when response object is unexpectedly nil
+Intercom::GatewayTimeoutError
 ```
 
 ### Rate Limiting