Add Ruby openapi3_parser

Author: kevindew

IMPLEMENTATIONS.md

@@ -15,6 +15,7 @@ These tools are not necessarily endorsed by the OAI.
 | swagger2openapi | [GitHub/mermade/swagger2openapi](https://github.com/mermade/swagger2openapi) | Node.js | An OpenAPI / Swagger 2.0 to OpenAPI 3.0.x converter and validator |
 | Tavis.OpenApi | [GitHub/tavis-sofware/Tavis.OpenApi](https://github.com/tavis-software/Tavis.OpenApi/) | dotnet | C# based parser with definition validation and migration support from V2 |
 | odata-openapi | [GitHub/oasis-tcs/odata-openapi](https://github.com/oasis-tcs/odata-openapi) | XSLT | OData 4.0 to OpenAPI 3.0.0 converter |
+| openapi3_parser | [GitHub/kevindew/openapi3_parser](https://github.com/kevindew/openapi3_parser) | Ruby | A Ruby implementation of parser and validator for the OpenAPI 3 Specification |
 
 
 #### Editors