Add documentation when running together with other LoadBalancer controllers

[jsiebens]

• I have raised an issue to propose this change.
  Fixes [Proposal] Promote and/or document the use of the Tunnel CRD #104

Description

Add a small section in the README on how to run together with another LoadBalancer controller like MetalLB

How Has This Been Tested?

Installed the inlets operator and exposed a service by using a Tunnel resource instead of a LoadBalancer service.

apiVersion: v1
kind: Service
metadata:
  name: nginx
spec:
  type: ClusterIP
  ports:
  - name: http
    port: 80
    targetPort: 80
  selector:
    app: nginx
---
apiVersion: inlets.inlets.dev/v1alpha1
kind: Tunnel
metadata:
  name: nginx
spec:
  serviceName: nginx
  auth_token: <token>

How are existing users impacted? What migration steps/scripts do we need?

No impact

Checklist:

I have:

• updated the documentation and/or roadmap (if required)
• read the CONTRIBUTION guide
• signed-off my commits with git commit -s
• added unit tests

[alexellis]

Approved. LGTM