Add a Github Security Policy

[whyisjake]

As part of the work that the Core Security team is undertaking, we should create a Security Policy for Github.

​https://help.github.com/en/github/managing-security-vulnerabilities/adding-a-security-policy-to-your-repository

This document will live at the root of wordpress-develop, and will serve as a public-facing document detailing the security policies for core.

[ayeshrajans]

Yay this is nice.
Shouldn't we link to the HackerOne page from the SECURITY.md file? ​https://hackerone.com/wordpress

[whyisjake]

In 46735:

Security: Add a GitHub Security Policy.

As part of more responsible security disclosure, we are adding a security policy to GitHub.

Fixes #48521.
Props whyisjake, ayeshrajans.

[ayeshrajans]

#47368 was marked as a duplicate.

[SergeyBiryukov]

In 46736:

Security: Add WordPress 5.3.x to the "Supported Versions" section of GitHub Security Policy.

Props imath.
Fixes #48667. See #48521.

[SergeyBiryukov]

In 47403:

Tests: Add a unit test to ensure the "Supported Versions" section of GitHub Security Policy always includes the latest stable branch.

See #48667, #48521.