Update users.mdx (#289)

Add Severalnines with link. Our CCX DBaaS platform supports CloudStack-based infra.

Author: ksbuzzell

src/pages/users.mdx

@@ -64,6 +64,7 @@ mailing list <b>users@cloudstack.apache.org</b>.
 - [SJC Inc](http://www.sjcloud.cn/index.xhtml)
 - [Server24](https://www.server24.eu)
 - [Serverion](http://www.serverion.com)
+- [Severalnines](https://severalnines.com)
 - [Telia Latvia](http://www.telia.lv)
 - [Tranquil Hosting](http://www.tqhosting.com)
 - [Treestle BV](http://www.liquidns.com)