add more info about vendor extensions

Author: wing328

CONTRIBUTING.md

@@ -52,6 +52,7 @@ You may find the current code base not 100% conform to the coding style and we w
 For [Vendor Extensions](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#vendorExtensions), please follow the naming convention below:
 - For general vendor extension, use lower case and hyphen. e.g. `x-is-unique`, `x-content-type`
 - For language-specified vendor extension, put it in the form of `x-{lang}-{extension-name}`. e.g. `x-objc-operation-id`, `x-java-feign-retry-limit` 
+- For a list of existing vendor extensions in use, please refer to https://github.com/swagger-api/swagger-codegen/wiki/Vendor-Extensions. If you've addaed new vendor extensions as part of your PR, please update the wiki page.
 
 ### Testing