Create a SECURITY.md to document security policies

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+For information regarding which versions of DSpace are currently under support, please see our DSpace Software Support Policy:
+
+https://wiki.lyrasis.org/display/DSPACE/DSpace+Software+Support+Policy
+
+## Reporting a Vulnerability
+
+If you believe you have found a security vulnerability in a supported version of DSpace, 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 see our DSpace Software Support Policy
+for information on privately reporting vulnerabilities:
+
+https://wiki.lyrasis.org/display/DSPACE/DSpace+Software+Support+Policy