Add SECURITY.md
This is a copy of the official version (https://gitlab.com/hpctoolkit/governance/-/blob/33c72d6baf10f135bc09950f6bdf3d068b6cc85b/SECURITY.md) but with minor adjustments to fit this repository.
This is a copy of the official version (https://gitlab.com/hpctoolkit/governance/-/blob/33c72d6baf10f135bc09950f6bdf3d068b6cc85b/SECURITY.md) but with minor adjustments to fit this repository.