Create SECURITY.md (pytorch#61356)

Summary: Pull Request resolved: pytorch#61356

Reviewed By: samestep

Differential Revision: D29589904

Pulled By: malfet

fbshipit-source-id: 5d79d25e35af9cb258fd6843559955360dc0cc4e

SECURITY.md

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+# Reporting Security Issues
+
+If you believe you have found a security vulnerability in PyTorch, we encourage you to let us know right away. We will investigate all legitimate reports and do our best to quickly fix the problem.
+
+Please refer to the following page for our responsible disclosure policy, reward guidelines, and those things that should not be reported:
+
+https://www.facebook.com/whitehat