Docs: add contact email for security issues

Author: medmunds

docs/contributing.rst

@@ -30,6 +30,8 @@ Bugs
 ----
 
 You can report problems or request features in `Anymail's GitHub issue tracker`_.
+(For a security-related issue that should not be disclosed publicly, instead email
+Anymail's maintainers at security<AT>anymail<DOT>info.)
 
 We also have some :ref:`troubleshooting` information that may be helpful.