Update README with revised security policy (#9553)

Author: radarhere

README.md

@@ -106,4 +106,8 @@ The core image library is designed for fast access to data stored in a few basic
 
 ## Report a vulnerability
 
-To report a security vulnerability, please follow the procedure described in the [Tidelift security policy](https://tidelift.com/docs/security).
+To report sensitive vulnerability information, report it [privately on GitHub](https://github.com/python-pillow/Pillow/security/advisories/new).
+
+If you cannot use GitHub, use the [Tidelift security contact](https://tidelift.com/security). Tidelift will coordinate the fix and disclosure.
+
+DO NOT report sensitive vulnerability information in public.