Create security self-assessment.md of k8gb and contribute it to CNCF Tag security

[ytsarev]

During security slam we were asked about contributing our self-assessment.md into https://github.com/cncf/tag-security/tree/main/assessments/projects

We can considerate the creation of such a document. There are some good examples in the repo.

Basic version of it was created during security slam and located at https://github.com/k8gb-io/k8gb/blob/master/self-assessment.md