Update "Security" section of README to also mention the Security Policy

I did this because some projects use a README section named  "Security" to disclose the same infos that we're disclosing on the Security Policy. After this change they'd be correctly redirected.

Author: diogoteles08

README.md

@@ -26,6 +26,8 @@ licensed under the [Apache License, version 2.0](https://www.apache.org/licenses
 
 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.
 
+For instructions on how to best report security issues, see our [Security Policy](https://github.com/sybrenstuvel/python-rsa/blob/main/SECURITY.md).
+
 ## Setup of Development Environment
 
 ```