Create SECURITY.md (#7054)

* Create SECURITY.md

Resolves https://github.com/ultralytics/yolov5/issues/7052

* Move into ./github

* Update SECURITY.md

.github/SECURITY.md

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+# Security Policy
+
+We aim to make YOLOv5 🚀 as secure as possible! If you find potential vulnerabilities or have any concerns please let us know so we can investigate and take corrective action if needed.
+
+### Reporting a Vulnerability
+
+To report vulnerabilities please email us at hello@ultralytics.com or visit https://ultralytics.com/contact. Thank you!