Noting support for OpenAPI v3

[earth2marsh]

… and some cleanup to other OpenAPI references.

(note: because this is a docs change, I didn't bother opening an issue—hope that's ok!)

[lafrech]

(No pb, you don't have to open an issue when you open a PR.)

OpenAPI v3 support is being worked on, but I'm not sure it should be advertised as "supported" already. @Bangertm, what do you think?

BTW, the work on this can be followed in #165.

[Bangertm]

It's advertised as:

A pluggable API specification generator. Currently supports the OpenAPI (versions 2.0 and 3.0) spec.

On marshmallow-code. The 3.0 support is fairly rudimentary though. It's basically what you get after running a 2.0 spec through a 2 to 3 converter tool. No support for any of the new features of 3.0 (such as polymorphism 😉).

[sloria]

Maybe the docs should read "Currently supports OpenAPI Specification version 2, with limited support for version 3" ?

[earth2marsh]

Thanks for clarifying. I've taken a pass to encorporate the suggested changes, HTH!

When more complete v3 support lands, would love to add you to the OpenAPI Implementations list.

[sloria]

Thanks!