Add Open Systems to adopters (#1753)

After the introduction of Istio support we are happy users of K8GB

Signed-off-by: Andre Baptista Aguas <andre.aguas@protonmail.com>

ADOPTERS.md

@@ -16,3 +16,4 @@ This list is sorted in the order that organizations were added to it.
   [Absa](https://www.absa.co.za/) | @kuritka | K8GB was created in Absa and had there first production use in cross-regional datacenter context
 [Millennium bcp](https://www.millenniumbcp.pt/) | @infbase | Multicloud and multi-region cloud native load balancing
 [Eficode](https://eficode.com/) | @punasusi | As a cloud-native and devops consulting firm, we have used k8gb in customer engagements
+[Open Systems](https://www.open-systems.com/) | @abaguas | Multi-cluster load balancing in private WAN