Add URL with more info to timing security issues

Author: sybrenstuvel

README.md

@@ -26,7 +26,7 @@ licensed under the [Apache License, version 2.0](https://www.apache.org/licenses
 Security
 --------
 
-Because of how Python internally stores numbers, it is very hard (if not impossible) to make a pure-Python program secure against timing attacks. This library is no exception, so use it with care.
+Because of how Python internally stores numbers, it is very hard (if not impossible) to make a pure-Python program secure against timing attacks. This library is no exception, so use it with care. See https://securitypitfalls.wordpress.com/2018/08/03/constant-time-compare-in-python/ for more info.
 
 
 Major changes in 4.1