docs: Add security policy

Author: XM5jDcsHTyGJtQqlCi

SECURITY.md

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+# Security Policy
+
+Security updates will be provided for the most recent minor releases of all packages.
+
+Updates for earlier releases may be considered, but are not guaranteed.
+
+To report a suspected security vulnerability,
+please do so on GitHub via this form:
+https://github.com/messageformat/messageformat/security/advisories/new
+
+You may expect a response within 48 hours.
+
+For extended support or a stronger guarantee,
+please reach out to the maintainers to discuss a support agreement.