docs: add SECURITY.md (#7693)

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you've discovered a security issue, please report it through the form linked
+below, which will create a secure, private ticket.
+https://bugzilla.mozilla.org/form.web.bounty
+
+MDN may be eligible for
+[Mozilla's Security Bug Bounty Program](https://www.mozilla.org/en-US/security/bug-bounty/).
+You can find more information about the bounty program in the
+[Mozilla Web Bug Bounty FAQ](https://www.mozilla.org/en-US/security/bug-bounty/faq-webapp/).
+You can use the above form even if you are not interested in a bounty reward.