DATAJDBC-382 - Create security policy readme.

SECURITY.adoc

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+# Security Policy
+
+## Supported Versions
+
+Please see the https://spring.io/projects/spring-data-jdbc[Spring Data JDBC] project page for supported versions.
+
+## Reporting a Vulnerability
+
+Please don't raise security vulnerabilities here. Head over to https://pivotal.io/security to learn how to disclose them responsibly.