Add LICENSE as per handbook guidelines

What

adds a license as outlined by handbook guidelines for new projects.

Context

dast-runner-validation is project that allows dast site validation to happen on a runner instead of inside a sidekiq worker. it achieves via the docker image that will be produced from the code in this project's repository. it is complimentary to the functionality offered by the existing dast project.

Why

all gitlab projects should carry a license.