Update `SECURITY.md` to indicate issues can be reported on Github (#26599)

SECURITY.md

@@ -1,8 +1,11 @@
 # Security Policy
 
-If you believe you've identified a security vulnerability in Mastodon (a bug that allows something to happen that shouldn't be possible), you can reach us at <security@joinmastodon.org>.
+If you believe you've identified a security vulnerability in Mastodon (a bug that allows something to happen that shouldn't be possible), you can either:
 
-You should _not_ report such issues on GitHub or in other public spaces to give us time to publish a fix for the issue without exposing Mastodon's users to increased risk.
+- open a [Github security issue on the Mastodon project](https://github.com/mastodon/mastodon/security/advisories/new)
+- reach us at <security@joinmastodon.org>
+
+You should _not_ report such issues on public GitHub issues or in other public spaces to give us time to publish a fix for the issue without exposing Mastodon's users to increased risk.
 
 ## Scope