docs: cluster draft

Signed-off-by: Anthony Griffon <[email protected]>

Author: Miaxos

docs/cluster.md

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+# Roster Cluster Specification
+
+First of all, Roster cluster is not a Redis Cluster. Even if you are able to
+interact with a Roster cluster in the same way as with a Redis Cluster, we don't
+implement the protocol allowing roster to be integrated inside a Redis Cluster.
+
+## Main goals
+
+- High performances
+- Availability
+- Handling of strongly replicated data and weakly replicated data.
+- ACL must be highly consistent
+
+### ACL Replication
+
+A Roster Cluster got his ACL globally replicated in every node through a Raft
+protocol.