Add documentation about how to support other IDL format (protobuf, ...)

At the moment, only openapis is supported, while it is enough to explore the potentiality of this generator code, we would like to explore how complicated it would be to bring support for protobuf for this. Once you feel confident enough in your design choices for openapi, could you add documentation so that community/contributor could start exploring coding a support for other IDL ? (Protobuf, thrift, ...)