README: update security reference

Now that we have security disclosure and release process in place,
update the related reference.

README.md

@@ -171,7 +171,7 @@ See [reporting bugs](Documentation/reporting_bugs.md) for details about reportin
 
 ## Reporting a security vulnerability
 
-A security vulnerability can be reported as an issue, however, GitHub and mailing lists may NOT always be an appropriate place for reporting vulnerabilities. In that case, please reach out to the project [MAINTAINERS](https://github.com/etcd-io/etcd/blob/master/MAINTAINERS) to first discuss the vulnerabilities with them and take necessary action per such discussion.
+See [security disclosure and release process](security/README.md) for details on how to report a security vulnerability and how the etcd team manages it.
 
 ## Issue and PR management