Update implementations.md

Added OpenAPI2JSON schema converter

Author: benjagm

implementations.md

@@ -290,6 +290,7 @@ the utility, and decided on a case-by-case basis.
 
 -   OpenAPI
     -   [JSON Schema to OpenAPI Schema](https://github.com/wework/json-schema-to-openapi-schema) _draft-04_ Draft-06 and -07 planned per README (MIT)
+    -   [OpenAPI specification to JSON Schema](https://github.com/instrumenta/openapi2jsonschema) Draft-07 (MIT)
 -   Orderly
     -   [Orderly](https://github.com/lloyd/orderly) (BSD-3-Clause)
 -   RAML