Create Reparenting.markdown

doc/Reparenting.markdown

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+Reparenting
+
+This document describes the reparenting features of Vitess. Reparenting is used when the master for a Shard is
+changing from one host to another. It can be triggered (for maintenance for instance) or happen automatically
+(based on the current master dying for instance).
+
+Two main types of reparenting supported by Vitess are Active Reparents (the Vitess toolchain is handing it all)
+and External Reparents (another tool is responsible for reparenting, and the Vitess toolchain just update its
+internal state.
+
+Active Reparents
+
+They are triggered by using the 'vtctl ReparentShard' command. See the help for that command.
+
+External Reparents
+
+In this part, we assume another tool has been reparenting our servers. We then trigger the
+'vtctl ShardExternallyReparented' command.