updated ajv description

implementations.md

@@ -64,7 +64,7 @@ Validators
     -   <a href="https://github.com/everit-org/json-schema" id="link-impl-everit-json-schema">json-schema (implementation based on the org.json API)</a> - *supports version 4* (Apache License 2.0)
     -   <a href="https://github.com/networknt/json-schema-validator" id="link-impl-networknt-json-schema">json-schema-validator</a> - *supports version 4* (Apache License 2.0)
 - JavaScript <a id="validator-javascript"></a>
-    -   <a href="https://github.com/epoberezkin/ajv" id="link-impl-ajv">ajv</a> for Node.js and the browser - *supports version 4, validation keywords and $data reference from [version 5 proposals](https://github.com/json-schema/json-schema/wiki/v5-Proposals)* (MIT)
+    -   <a href="https://github.com/epoberezkin/ajv" id="link-impl-ajv">ajv</a> for Node.js and browsers - *supports version 4, version 6, [custom keywords](https://github.com/epoberezkin/ajv-keywords) and [$data reference](https://github.com/json-schema-org/json-schema-spec/issues/51)* (MIT)
     -   <a href="https://github.com/korzio/djv" id="link-impl-djv">djv</a> for Node.js and browsers - *supports version 4* (MIT)
     -   <a href="https://github.com/tdegrunt/jsonschema" id="link-impl-tdegrunt-jsonschema">jsonschema</a> for Node.js - *supports version 4* (MIT)
     -   <a href="https://github.com/mafintosh/is-my-json-valid" id="link-impl-is-my-json-valid">is-my-json-valid</a> - *supports version 4* (MIT)