add plusserver to adopters (coredns#6565)

Signed-off-by: Johann Gnaucke <[email protected]>

ADOPTERS.md

@@ -32,4 +32,5 @@
 * [BackMarket](https://www.backmarket.com) uses CoreDNS within Kubernetes in production, with standard configuration.
 * [Absa Group](https://www.absa.africa) uses CoreDNS as an integral part of Kubernetes Global Balancer project - [k8gb](https://www.k8gb.io/).
 * [Northflank](https://northflank.com/) uses CoreDNS on all of our Kubernetes clusters across GCP, AWS, and bare-metal.
-* [PITS Global Data Recovery Services](https://www.pitsdatarecovery.net) uses CoreDNS on K8s in its highly-loaded internal infrastructure. 
+* [PITS Global Data Recovery Services](https://www.pitsdatarecovery.net) uses CoreDNS on K8s in its highly-loaded internal infrastructure.
+* [plusserver](https://www.plusserver.com) uses CoreDNS on K8s in its plusserver Kubernetes Engine.