Add security policy

SECURITY.md

+## Supported Versions
+
+Only the docker image with the latest version of GHC is supported with security
+updates.
+
+## Reporting a Vulnerability
+
+To report a vulnerability in the latest docker image, email the maintainer
+olivier.benz@b-data.ch.
+
+## Vulnerabilities in Previous Versions
+
+Vulnerabilities in docker images with previous versions of GHC are not fixed.
+
+Whenever a new version of GHC is released, the previous version's docker image
+is rebuilt once again and then frozen.