add github repo security policy to github-action #905

Hi team:

As part of security review of githug-action, it is strongly recommended that github-action repo has security policy defined. common practice is also documented https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository.

Adding a security policy does demonstrate team's commitment on actively dealing with security issues, thus give user confidence for adoption.

Please consider add Security policy to make cypress github-action security strong.