Included links to previous versions

There should be an easy route for people to find the previous versions, as people will probably still be working with draft-4 right now. Sort of fixes json-schema-org/json-schema-spec#120 for me, given that I opened it.

documentation.html

@@ -23,6 +23,8 @@ <h2>Specification</h2>
     </table>
     <p>They are also available on the IETF main site: <a href="http://tools.ietf.org/html/draft-wright-json-schema-00">core (draft-wright-json-schema-00)</a>,  <a href="http://tools.ietf.org/html/draft-wright-json-schema-validation-00">validation (draft-wright-json-schema-validation-00)</a> and <a href="http://tools.ietf.org/html/draft-wright-json-schema-hyperschema-00">hyper-schema (draft-wright-json-schema-hyperschema-00)</a>.
     </p>
+    <p>For previous versions of the specification, please see the <a href="https://github.com/json-schema-org/json-schema-spec/wiki/Specification-Links">Specification Links</a> page on our github wiki.
+    </p>
 </div>
 
 <h2>Meta-schemas</h2>