allow OpenAPI 3.0

[pvdbosch]

The claim in the guide that OpenAPI 3.0 isn’t supported yet in most tooling is outdated. The API Gateway product that’s offered on gCloud doesn’t offer full support of OpenAPI 3.0 yet however but it is planned.
I don't think this should hold back those already ready for OpenAPI 3.0 however like FPS Mobility.
Customers of OpenAPI 3.0 could use tools to convert to OpenAPI 2.0.

So I propose to also allow 3.0, but add a warning that the guide isn’t optimized for it yet (#31). Most should still apply however.

[pvdbosch]

eHealth uses https://www.npmjs.com/package/api-spec-converter to convert from OpenAPI 2.0 to 3.0, because it gives better results with code generation. Some manipulation on its input 2.0 file may be required for it to work correctly.

[pvdbosch]

updated master to allow OpenAPI 3.0