USERS.md: Add Santa Claus to the list of users

For Christmas 2023, Santa and the Elves switched to Cilium. They're
delighted with the performance improvements and the acceleration of the
Christmas tour, and would like to appear in the list of satisfied users.

Link: https://qmonnet.github.io/whirl-offload/2024/01/02/santa-switches-to-cilium/
Signed-off-by: Quentin Monnet <quentin@isovalent.com>

USERS.md

@@ -537,6 +537,11 @@ Users (Alphabetically)
       U: CNI, Overlay networking
       Q: @Madhu CS
 
+    * N: Santa Claus & the Elves
+      D: All our infrastructure to process children's letters and wishes, toy making and delivery, distributed over multiple clusters around the world, is now powered by Cilium.
+      U: ClusterMesh, L4LB, XDP acceleration, Bandwidth manager, Encryption, Hubble
+      L: https://qmonnet.github.io/whirl-offload/2024/01/02/santa-switches-to-cilium/
+
     * N: SAP
       D: SAP uses Cilium for multiple internal scenarios. For examples for self-hosted Kubernetes scenarios on AWS with SAP Concur and for managed Kubernetes clusters provided with Gardener Project across AWS, Azure, GCP, and OpenStack.
       U: CNI , Overlay Network, Network Policies