Add yii2-openapi to OpenAPI Generator

[cebe]

moved here from cebe#10

Hey, there!

I'm PHP contributor of OpenAPI Generator project. I used OAS3 schema with generated PHP server for a few years already and just noticed your package.

Maybe you should integrate your server solution to OpenAPI Generator? Here is a brief guide - Create a New Generator

cc core members of OpenAPI Generator @wing328 and @jimschubert