Add specific retry codes in README section

Author: nikochiko

README.md

@@ -128,7 +128,9 @@ A request is deemed retriable when any of the following HTTP status codes is ret
 
 - [408](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/408) (Timeout)
 - [429](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/429) (Too Many Requests)
-- [5XX](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/500) (Internal Server Errors)
+- [502](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/502) (Bad Gateway)
+- [503](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/503) (Service Unavailable)
+- [504](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/504) (Gateway Timeout)
 
 Use the `max_retries` request option to configure this behavior.