cleanup: Tagged README examples (v3.0)

[pcolledg-amd]

Tracking major/minor tags (v3/v3.0) for default branch commits so we can regulate enthusiastic user access. This PR suggests minor tag but major would be fine with me too.

Release snapshot tags (v3.0.0, ie. revision+) won't change and will be recommended in Release Notes.

Examples:

kubernetes-onload$ oc apply -k 'https://github.com/Xilinx-CNS/kubernetes-onload/config/default?ref=v3.0'
error: git cmd = '/usr/bin/git fetch --depth=1 origin v3.0': exit status 128

kubernetes-onload$ git tag v3.0 && git push origin v3.0
kubernetes-onload$ oc apply -k https://github.com/pcolledg-amd/kubernetes-onload/config/default?ref=v3.0
namespace/onload-operator-system unchanged
Warning: resource customresourcedefinitions/onloads.onload.amd.com is missing the kubectl.kubernetes.io/last-applied-configuration annotation which is required by oc apply. oc apply should only be used on resources created declaratively by either oc create --save-config or oc apply. The missing annotation will be patched automatically.
customresourcedefinition.apiextensions.k8s.io/onloads.onload.amd.com configured
serviceaccount/onload-operator-controller-manager unchanged
role.rbac.authorization.k8s.io/onload-operator-leader-election-role unchanged
clusterrole.rbac.authorization.k8s.io/onload-operator-manager-role unchanged
clusterrole.rbac.authorization.k8s.io/onload-operator-metrics-reader unchanged
clusterrole.rbac.authorization.k8s.io/onload-operator-proxy-role unchanged
rolebinding.rbac.authorization.k8s.io/onload-operator-leader-election-rolebinding unchanged
clusterrolebinding.rbac.authorization.k8s.io/onload-operator-manager-rolebinding unchanged
clusterrolebinding.rbac.authorization.k8s.io/onload-operator-proxy-rolebinding unchanged
service/onload-operator-controller-manager-metrics-service unchanged

$ git clone -b v3.0 https://github.com/pcolledg-amd/kubernetes-onload `mktemp -d`
Cloning into '/tmp/tmp.8vHdBcGR8d'...
...
You are in 'detached HEAD' state. ...

[abower-amd]

I don't understand why moving tags are better than branches?