Add security policy.

SECURITY.md

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+# SECURITY
+
+It's somewhat unlikely that tcell is in a security sensitive path,
+but we do take security seriously.
+
+## Vulnerabilityu Response
+
+If you report a vulnerability, we will respond within 2 business days.
+
+## Report a Vulnerability
+
+If you wish to report a vulnerability found in tcell, simply send a message
+to garrett@damore.org.  You may also reach us on our discord channel -
+https://discord.gg/urTTxDN - a private message to `gdamore` on that channel
+may be submitted instead of mail.