Create SECURITY.md

Closes #778

Author: valenting

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 2.2.x   | :white_check_mark: |
+
+## Reporting a Vulnerability
+
+The most secure way to report a vulnerability while this crate is maintained by a Mozilla contributor is by reporting a bug on the Bugzilla issue tracker using this [link](https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&component=Networking&groups=network-core-security&short_desc=[rust-url]%20DESCRIPTION&bug_type=defect).
+
+Alternatively, you may send an email to valentin AT mozilla DOT com.