Add security policy

SECURITY.md

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+# Security Policy
+
+If you believe you have found a security critical bug, please **DO NOT** open a regular issue and instead follow the reporting steps below.
+For all regular bugs (e.g. incorrect results) please feel free to open an issue on the github page.
+
+## Reporting a Vulnerability
+
+Please email your security bugs to matt at mattborland dot com.
+I will respond same day, or next business day (I am located in the Eastern US time zone) and work with you to diagnose and fix the issue.
+My public key is listed below so that you can securely email me:
+
+```
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+
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+waBi3m2PJHLNK21hdHRAbWF0dGJvcmxhbmQuY29tIDxtYXR0QG1hdHRib3Js
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+Ah4BACEJEFmBWlVCFaWlFiEEwTgurTcoHwbdSbIsWYFaVUIVpaVWuwEA77rm
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+EkoDFlNzQZqNldbjRcPyI5UH
+=462S
+-----END PGP PUBLIC KEY BLOCK-----
+```