docs: add Limitations section on README

Add a section describing how this charm can only be used as an OIDC client when Dex and
Charmed Kubeflow are deployed in the model; otherwise it will fail, as there are multiple
parts of this charm's code that are tightly coupled to those.

Fixes #159

README.md

@@ -10,12 +10,18 @@ This repository hosts the Kubernetes Python Operator for OIDC Gatekeeper
 
 The OIDC Gatekeeper Operator may be deployed using the Juju command line as follows
 ```bash
-juju deploy oidc-gatekeeper
-juju config oidc-gatekeeper client-secret=<client-secret> public-url=http://<public-url>
+juju deploy oidc-gatekeeper --trust
+juju deploy dex-auth --trust
+juju config oidc-gatekeeper client-secret=<client-secret, optional>
+juju integrate dex-auth:dex-oidc-config oidc-gatekeeper:dex-oidc-config
 ```
 
 Upstream documentation can be found at https://github.com/arrikto/oidc-authservice
 
+## Limitations
+
+This charm has been designed around Charmed Kubeflow and it will not work as an OIDC client outside of a model where `dex-auth` and Charmed Kubeflow are deployed. There are currently no plans to change this behaviour.
+
 ## Looking for a fully supported platform for MLOps?
 
 Canonical [Charmed Kubeflow](https://charmed-kubeflow.io) is a state of the art, fully supported MLOps platform that helps data scientists collaborate on AI innovation on any cloud from concept to production, offered by Canonical - the publishers of [Ubuntu](https://ubuntu.com).