cleaned up documentation, round 1

notes/cluster_notes.md

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+### How to choose shards, replicas and cluster size: Rules of Thumb.
+
+sh      = shards
+rf      = replication factor. replicas = 0 implies rf = 1, or 1 replica of each shard.
+
+pm      = running data_esnode processes per machine
+N       = number of machines
+
+n_cores = number of cpu cores per machine
+n_disks = number of disks per machine
+
+* You must have at least as many data_esnodes as 
+  Mandatory:  (sh * rf) < (pm * N)
+
+  Shards:     shard size < 10GB
+
+More shards = more parallel writes