add a maxver attribute to enum values

This could allow deprecating enum values.

EBMLSchema.xsd

@@ -140,6 +140,7 @@
     <xs:attribute name="label"/>
     <xs:attribute name="value" use="required"/>
     <xs:attribute name="minver"/>
+    <xs:attribute name="maxver"/>
   </xs:complexType>
 
   <xs:complexType name="documentationType" mixed="true">

specification.markdown

@@ -1168,6 +1168,21 @@ The `minver` attribute is **OPTIONAL**. If the `minver`
 attribute is not present, then the enum value has the same value as the `minver` attribute of the
 EBML Element it belongs to.
 
+#### maxver
+
+Within an EBML Schema, the XPath of the `@value` attribute is
+`/EBMLSchema/element/restriction/enum/@maxver`.
+
+The `maxver` (maximum version) attribute stores a nonnegative integer that
+represents the last or most recent version of the docType to support
+the enum value. `maxver` **MUST** be greater than or equal to 
+the first version of the docType to support the enum value, whether it's set
+in the `minver` attribute of the enum or implied.
+
+The `maxver` attribute is **OPTIONAL**. If the `maxver` attribute is
+not present, then the enum value's maximum version is the value 
+stored in the `version` attribute of <EBMLSchema>.
+
 ### `<extension>` Element
 
 Within an EBML Schema, the XPath of the `<extension>`