Indicate that required can take an empty array.

Author: mdboom

source/reference/object.rst

@@ -193,9 +193,14 @@ By default, the properties defined by the ``properties`` keyword are
 not required.  However, one can provide a list of required properties
 using the ``required`` keyword.
 
-The ``required`` keyword takes an array of one or more strings.  Each
+The ``required`` keyword takes an array of zero or more strings.  Each
 of these strings must be unique.
 
+.. draft_specific::
+
+   --Draft 4
+   In Draft 4, ``required`` must contain at least one string.
+
 In the following example schema defining a user record, we require
 that each user has a name and e-mail address, but we don't mind if
 they don't provide their address or telephone number: