Put in Security link in repo README.

For the repos we're assigned to we should ensure that in the README.md that we put in an explicit link as to where to report security issues.

GitHub has a secure feature to manage security advisories.

We should determine if we want all advisories are tracked in the Entities repo, or the product repos.
For those that don't read the doco, they'll just put them in the product repo.

I expect a slew of these as people look for places to try out AI assisted security tools.

Review the assigned repositories' README.md files and GitHub's security advisory guidance first. Resolve whether advisories belong in the Entities repository or individual product repositories, then add an explicit security-reporting link to each applicable README. Done means the policy is documented and users can find where to report security issues.

Written by the indexing model from the issue text.