Use standard Spring code of conduct

Closes gh-565.

Author: mp911de

CODE_OF_CONDUCT.adoc

@@ -1,9 +1,9 @@
-# Security Policy
+= Security Policy
 
-## Supported Versions
+== Supported Versions
 
 Please see the https://spring.io/projects/spring-vault[Spring Vault] project page for supported versions.
 
-## Reporting a Vulnerability
+== Reporting a Vulnerability
 
 Please don't raise security vulnerabilities here. Head over to https://pivotal.io/security to learn how to disclose them responsibly.