Add a security policy

SECURITY.md

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+# Security Policy
+
+If security issues arise, we will try to fix those as soon as possible.
+
+Due to Parso's nature, Security Issues will probably be extremely rare, but we will of course treat them seriously.
+
+## Reporting Security Problems
+
+If you need to report a security vulnerability, please send an email to davidhalter88@gmail.com. Typically, I will respond in the next few business days.