add example using validator - closes #129

[clux]

Time to put the ancient issue to rest now that the ecosystem has done all the work for us :-)

Minor expected caveat:

if you add #[validate(custom = "custom_validator_fn")] that does nothing to the schema.

not sure if that's worth warning about or not.

[nightkr]

I'd add a warning about that, yeah. Also worth mentioning that this validation only applies when modifying the object on the API side, so if you change the validation rules then you can't rely on old objects obeying them.

[clux]

Added more docs; caveats when using validator and a segment about versioning (linking to the open issue).