Add security policy in SECURITY.md

Author: ronf

SECURITY.md

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+# Secuity Policy
+
+## Supported Versions
+
+AsyncSSH has only one active development branch at this time. Any bug or
+vulnerability fixes will be fixed in the "develop" branch first and then
+migrated to the "master" branch in preparation for putting out a new release.
+
+## Reporting Vulnerabilities
+
+**⚠️ Please do not file GitHub issues for security vulnerabilities as they are
+public! ⚠️**
+
+If you believe you have found a security vulnerability in AsyncSSH, please
+draft a new
+[security advisory](https://github.com/ronf/asyncssh/security/advisories/new)
+or send an e-mail to security@asyncssh.com with a description of the issue
+and details for how to reproduce it. This report will be reviewed and you'll
+be contacted if further information is required, or when a fix is available.
+
+Published security advisories for AsyncSSH can be found
+[here](https://github.com/ronf/asyncssh/security).