Add Zendesk to Opencost ADOPTERS.md

We are using opencost as part of our k8s cost attribution system v2. 

The previous version of the k8s cost attribution system was built in-house and had many limitations that were improved by switching to opencost on the new version. 

We use opencost API only, we don't use opencost-ui, since we have our internal data processing pipeline and visualization system for all our cloud spending and budget tracking. 

Signed-off-by: Leonardo Rodrigues de Mello <lrodriguesdemello@zendesk.com>

ADOPTERS.MD

@@ -14,3 +14,4 @@ If you would like to be included in this table, please submit a PR to this file
 | Grafana Labs                               | *                                 | end user               | [How Grafana Labs uses and contributes to OpenCost](https://grafana.com/blog/2023/02/02/how-grafana-labs-uses-and-contributes-to-opencost-the-open-source-project-for-real-time-cost-monitoring-in-kubernetes/) |
 | Microsoft                                  | *                                 | Service Provider       | [Leverage OpenCost on Azure Kubernetes Service](http://aka.ms/aks/OpenCost-AKS) |
 | mindcurv group                             | *                                 | Consultancy            | [mindcurv group](https://mindcurv.com/en/) |
+| Zendesk                                    | *                                 | end user               | [Zendesk](https://www.zendesk.com/) |