Add SECURITY file

SECURITY.rst

+###############
+Security Policy
+###############
+
+*************************
+Reporting a Vulnerability
+*************************
+
+Help keep our infrastructure safe! If you discover a vulnerability,
+please immediately get in touch with techsupport@scipost.org and
+Jean-Sébastien Caux jscaux@scipost.org so we can quickly patch it.