Create a Security Policy (#222)

* Create SECURITY.md

* Update SECURITY.md to add email

* Disclose vulnerability on Github

Author: joycebrum

SECURITY.md

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+# Reporting Security Issues
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+To report a security issue, please disclose it at [security advisory](https://github.com/certifi/python-certifi/security/advisories/new).
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+We will respond within 7 working days of your submission. If the issue is confirmed as a vulnerability, we will open a Security Advisory and acknowledge your contributions as part of it. This project follows a 90 day disclosure timeline.
+