Add instructions for reporting security issues.

Author: plusvic

SECURITY.md

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+Reporting security issues
+=========================
+
+The YARA authors try their best to keep this software free of security issues,
+but 100% secure software does not exist. We appreciate all kinds of bug reports,
+but those that constitute a security vulnerability are specially welcomed and
+they will be prioritized and publicly acknowledged.
+
+ Please do not open a GitHub issue to report a security vulnerability. If you
+ believe you have found a security vulnerability, please submit your report to
+
+ ```
+ [email protected]
+ ```
+
+We'll get back to you as soon as possible (usually in 1-2 days).