Add security policy

SECURITY.md

+## Supported Versions
+
+THE PROGRAM IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.
+
+The docker images are built from
+[tagged Git LFS releases](https://github.com/git-lfs/git-lfs/tags).
+
+## Reporting a Vulnerability
+
+See [Security Policy · git-lfs/git-lfs](https://github.com/git-lfs/git-lfs/security/policy).