Add SECURITY.md (#742)

Author: trivikr

.github/SECURITY.md

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+## Reporting Security Issues
+
+We take all security reports seriously.
+When we receive such reports,
+we will investigate and subsequently address
+any potential vulnerabilities as quickly as possible.
+If you discover a potential security issue in this project,
+please notify AWS/Amazon Security via our
+[vulnerability reporting page](http://aws.amazon.com/security/vulnerability-reporting/)
+or directly via email to [AWS Security](mailto:[email protected]).
+Please do _not_ create a public GitHub issue in this project.