Merge pull request ipython#28 from Carreau/secmd

Add security.md

Author: Carreau

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+All IPython and Jupyter security are handled via security@ipython.org. 
+You can find more information on the Jupyter website. https://jupyter.org/security
+
+## Tidelift
+
+You can report security concerns for IPython via the [Tidelift platform](https://tidelift.com/security).