Further library maintenance

@cebe Please make a decision and/or statement on whether:

- this library is abandoned by you, and you are unsure what to do about it
- the library is abandoned, and you will assign a maintainer for it
- or you are busy at the moment, but will return to maintaining the library in some foreseeable future
- if you plan to further develop this lib only if you get a payment for it, and do not consider having other maintainers (but then it's IMO the worst scenario, the community will just gradually switch to a fork)

the library is abandoned, please consider negotiating a maintainer for it, or point to the fork which will officially become the successor of this lib, although IMO it's better to keep the original library evolving.

If you fear that the new maintainer will not keep up to the same quality standards as you have in mind, give them some rough guidelines, it should not take too much of your time. Also you could do some occasional reviews if you wish.

There is a guy who already maintains [a fork](https://github.com/DEVizzent/cebe-php-openapi), he might be a good candidate, and I can suggest myself too!

The community can cope on its own, but it would be MUCH nicer and convenient if the transition process was official. Thank you!