Create Security.md file to show on GitHub Security/Policy page

Create Security.md file to show on GitHub Security/Policy page as default policy

SECURITY.md

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+# Reporting Security Issues
+
+Magento values the contributions of the security research community, and we look forward to working with you to minimize risk to Magento merchants. 
+
+## Where should I report security issues?
+
+We strongly encourage you to report all security issues privately via our [bug bounty program](https://hackerone.com/magento).  Please provide us with relevant technical details and repro steps to expedite our investigation.  If you prefer not to use HackerOne, email us directly at `[email protected]` with details and repro steps.  
+
+## Learning More About Security
+To learn more about securing a Magento store, please visit the [Security Center](https://magento.com/security).