Add security policy (#1388)

SECURITY.md

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+## Reporting Security Issues
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+If you believe you have found a security vulnerability in OpenPGP.js, please report it via email to [security@openpgpjs.org](mailto:security@openpgpjs.org). If possible, encrypt your message with our PGP key: it can be downloaded automatically using [WKD](https://wiki.gnupg.org/WKD), or manually on [openpgpjs.org](https://openpgpjs.org/.well-known/openpgpkey/hu/t5s8ztdbon8yzntexy6oz5y48etqsnbb?l=security).
+
+You should receive a response within 2 working days.