Add SECURITY.md (copied from https://github.com/vector-im/.github/blob/main/SECURITY.md)

Author: bmarty

SECURITY.md

@@ -0,0 +1,5 @@
+# Reporting a Vulnerability
+
+**If you've found a security vulnerability, please report it to security@matrix.org**
+
+For more information on our security disclosure policy, visit https://www.matrix.org/security-disclosure-policy/