Add OpenUPM to Verdaccio use cases (#4673)

[OpenUPM](https://openupm.com) is a fascinating use case of Verdaccio for something completely unrelated to node or npm: A public website and repository for [Unity](https://unity.com) packages which is interactive, real-time 3D content.

website/docs/who-is-using.md

@@ -28,6 +28,7 @@ _If you are using Verdaccio in your business and want to share your experience,
 - [Hyperledger Caliper](https://github.com/hyperledger/caliper)
 - [secco](https://secco.lekoarts.de)
 - [Nx](https://nx.dev/)
+- [OpenUPM](https://openupm.com/docs/)
 
 #### Readme Recommendations {#readme-recommendations}