docs: Update instructions on vulnerability reporting

Author: eemeli

docs/SECURITY.md

@@ -5,7 +5,9 @@ Security updates will be provided for the most recent minor v1 and v2 releases o
 No updates are provided for earlier releases.
 
 To report a suspected security vulnerability,
-please send an email to the maintainer at **eemeli@gmail.com**.
+please do so on GitHub via this form:
+https://github.com/eemeli/yaml/security/advisories/new
+
 You may expect a response within 48 hours.
 
 For extended support or a stronger guarantee,