Api: update the wiki about the transformer

[Grummfy]

Hello,
I think adding some word about how to create a transformer (like extending LeagueFractalTransformerAbstract or read https://fractal.thephpleague.com/transformers/ for more information) could be really helpfull and make people win a lot of time

when I see issue like #1219 I'm suppossing I'm not the same to ask this question ;)

[catalinux]

I think it says: https://github.com/dingo/api/wiki/Transformers

Fractal

Fractal is the default transformation layer used by the package. It boasts a number of useful features > to help you keep your data consistent.

To use Fractal it's recommend you read through the documentation found on their website.

[Grummfy]

Yes, but having a bit more information, doesn't cost that much. Saying which class to extends.... Having a little help, make a lot of difference

[kirkbushell]

@Grummfy I agree. nearly 3 years later, encountered the same issue - I'm actually stuck setting up transformers as I'm seeing errors but I've followed the wiki docs. Little bit frustrating.

[specialtactics]

I'm open to suggestions on things to add, but there's already a lot of info there.