Add firewall rules and ing class clarifications

[pdoreau]

This change is

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[bprashanth]

please surface this in the requirements section instead, something like:

if you're running multiple ingress controllers, or running on a cloudprovider that natively handles 
ingress, you need to specify the annotation `kubernetes.io/ingress.class: "nginx"` in all ingresses 
that you would like this controller to claim. Not specifying the annotation will lead to multiple 
ingress controllers claiming the same ingress. Specifying the wrong value will result in all ingress 
controllers ignoring the ingress. Multiple ingress controllers running in the same cluster was not 
supported in Kubernetes versions < 1.3.

[pdoreau]

@bprashanth fine to me, updated with your suggestion

[bprashanth]

please quote this, ie kubernetes.io/ingress.class: "nginx"

[bprashanth]

LGTM just one nit

[bprashanth]

LGTM, please sign CLA

[pdoreau]

I signed it!

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[k8s-github-robot]

Automatic merge from submit-queue