ADOPTERS: add pusher

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

ADOPTERS.md

@@ -4,3 +4,4 @@ This is a list of production adopters of Dex (in alphabetical order):
 
 - [Banzai Cloud](https://banzaicloud.com) is using Dex for authenticating to its Pipeline control plane and also to authenticate users against provisioned Kubernetes clusters (via Kubernetes OIDC support).
 - [Chef](https://chef.io) uses Dex for authenticating users in [Chef Automate](https://automate.chef.io/). The code is Open Source, available at [`github.com/chef/automate`](https://github.com/chef/automate).
+- [Pusher](https://pusher.com) uses Dex for authenticating users across their Kubernetes infrastructure (using Kubernetes OIDC support) in conjunction with the [OAuth2 Proxy](https://github.com/pusher/oauth2_proxy) for protecting web UIs.