Update SECURITY.md (#5448)

Link to GitHub security advisory process + PGP key

SECURITY.md

@@ -2,4 +2,5 @@
 
 ## Reporting a Vulnerability
 
-Please report security issues to alexandre@alapetite.fr
+Draft a [new security advisory](https://github.com/FreshRSS/FreshRSS/security/advisories) online,
+or report security issues to <alexandre@alapetite.fr> ([PGP public key if relevant](https://alexandre.alapetite.fr/cv/pgp.asc)).