Add relevant Github issues to Security Considerations

[Malvoz]

That's why we link to the issue so that people can discuss such things there (i.e. that's a good comment for the relevant issue).

insecure contexts are the low barrier to entry for kids learning to code with HTML

If you're referring to http://localhost environments, that may not be an issue:

user agents MAY treat localhost names as having potentially trustworthy origins

‐ https://w3c.github.io/webappsec-secure-contexts/#localhost