MXS-1354: Make new users backwards compatible

The `add user` and `enable account` commands create fully privileged
administrative users like they did in 2.1. This makes the addition of
read-only users backwards compatible.

Updated and expanded the documentation on administrative interface
users. Added entries into the release notes as well as the upgrading
document about relevant changes between 2.1 and 2.2.

Documentation/Release-Notes/MaxScale-2.2.0-Release-Notes.md

@@ -17,6 +17,19 @@ Significant whitespace in object names is now deprecated. All object names
 squeezing repeating whitespace and replacing it with hyphens. If any
 object name conversions take place, a warning will be logged.
 
+### Read-only Administrative Users
+
+Users that can only perform read-only operations can be created with `add
+readonly-user` and `enable readonly-account` commands.  To convert
+administrative users to read-only users, delete the old administrative user and
+create it as a read-only user.
+
+For more information about administrative interface users, refer to the
+[MaxAdmin](../Reference/MaxAdmin.md) documentation.
+
+**Note:** Old users from pre-2.2 MaxScale versions will be converted to
+administrative users that have full access to all commands.
+
 ### MaxAdmin
 
 The `remove user` command now only expects one parameter, the username.