docs: deploy tidb cluster in minikube

[cofyc]

Feature Request

Is your feature request related to a problem? Please describe:

Guide the user to use tidb-operator in the minikube environment.

Describe the feature you'd like:

Guide on how to set up and use tidb-operator in the minikube environment.

Describe alternatives you've considered:

NONE

Teachability, Documentation, Adoption, Migration Strategy:

[cofyc]

notes from @tennix

git clone https://github.com/pingcap/tidb-operator
cd tidb-operator
minikube start
kubectl apply -f manifests/tiller-rbac.yaml
helm init
helm install charts/tidb-operator --name tidb-operator --namespace tidb-admin
helm install charts/tidb-cluster --name tidb-cluster --namespace tidb --set schedulerName=default-scheduler,pd.storageClassName=standard,tikv.storageClassName=standard,pd.replicas=1,tikv.replicas=1,tidb.replicas=1
minikube service -n tidb tidb-cluster-grafana # this will open grafana with your default browser
minikube service -n tidb tidb-cluster-tidb --url  # connect with MySQL client using the first IP and Port