add semi manual process & readme for cluster quorum lost

[shyimo]

In Redis Cluster, there needs to be a majority of leader nodes to upgrade a slave to a master in case we lost some node(s).
At the moment we are not able to semi recover (with partial data lose) and we need to delete the all cluster in case we lost a majority of leader nodes.

We need to develop & document a process that will able to recover the cluster with partial data lose but without fully delete the cluster