Add security contact information

As per Tidelift's requirements.

Author: evansd

SECURITY.rst

@@ -0,0 +1,8 @@
+Security contact information
+============================
+
+
+To report a security vulnerability, please use the `Tidelift security contact
+<https://tidelift.com/security>`_.
+
+Tidelift will coordinate the fix and disclosure.