Add Upgrading to 2.1 document

The document lists important changes that should be noted when MaxScale is
upgraded from 2.0 to 2.1.

Documentation/Upgrading/Upgrading-To-MaxScale-2.1.md

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+# Upgrading MariaDB MaxScale from 2.0 to 2.1
+
+This document describes particular issues to take into account when upgrading
+MariaDB MaxScale from version 2.0 to 2.1.
+
+For more information about MariaDB MaxScale 2.1, please refer to the
+[ChangeLog](../Changelog.md).
+
+For a complete list of changes in MaxScale 2.1.0, refer to the
+[MaxScale 2.1.0 Release Notes](../Release-Notes/MaxScale-2.1.0-Release-Notes.md).
+
+## Installation
+
+Before starting the upgrade, we **strongly** recommend you back up your current
+configuration file.
+
+## MaxScale Log Files
+
+The name of the log file was changed from _maxscaleN.log_ to _maxscale.log_. The
+default location for the log file is _/var/log/maxscale/maxscale.log_.
+
+Rotating the log files will cause MaxScale to reopen the file instead of
+renaming them. This makes the MaxScale logging facility _logrotate_ compatible.
+
+## ReadWriteSplit
+
+The `disable_sescmd_history` option is now enabled by default. This means that
+slaves will not be recovered mid-session even if a replacement slave is
+available. To enable the legacy behavior, add the `disable_sescmd_history=true`
+parameter to the service definition.
+
+## Persistent Connections
+
+The MySQL session state is reset in MaxScale 2.1 for persistent
+connections. This means that any modifications to the session state (default
+database, user variable etc.) will not survive if the connection is put into the
+connection pool. For most users, this is the expected behavior.
+
+## User Data Cache
+
+The location of the MySQL user data cache was moved from
+`/var/cache/maxscale/<Service>` to `/var/cache/maxscale/<Service>/<Listener>`.
+
+## Galeramon Monitoring Algorithm
+
+Galeramon will assign the master status *only* to the node which has a
+_wsrep_local_index_ value of 0. This will guarantee consistent writes with
+multiple MaxScales but it also causes slower changes of the master node.
+
+To enable the legacy behavior, add `root_node_as_master=false` to the Galera
+monitor configuration.
+
+## MaxAdmin Editing Mode
+
+The default editing mode was changed from _vim_ to _emacs_ mode. To start
+maxadmin in the legacy mode, use the `-i` option.