Add the "example" keyword.

This addresses issue #121.

Most of the wording is lifted from "default", which is similar and can serve
the same purpose.  This wording explicitly allows for using "default" as an
example which was not part of the original request but seems reasonable.

Obviously, that part can be removed if deemed undesirable.

Author: handrews

jsonschema-validation.xml

@@ -627,6 +627,25 @@
                 </t>
             </section>
 
+            <section title='"example"'>
+                <t>
+                    There are no restrictions placed on the value of this keyword.
+                </t>
+                <t>
+                    This keyword can be used to provide a sample JSON value associated with a
+                    particular schema, for the purpose of illustrating usage.  It is
+                    RECOMMENDED that this value be valid against the associated schema.
+                </t>
+                <t>
+                    If this keyword is not present, but a "default" is specified,
+                    implementations MAY use the "default" value as if it were given
+                    as an example.
+                </t>
+                <t>
+                    This keyword MAY be used in root schemas, and in any subschemas.
+                </t>
+            </section>
+
         </section>
 
         <section title='Semantic validation with "format"'>