Custom validators

[vBazilevich]

Hello!

Could you please add a documentation entry on custom validators? Suddenly, not all validity constraints can be implemented using provided facilities. Such documentation will significantly simplify the process, which currently relies on implementation of custom struct with validate method returning rfl::Result. Also, it seems to be a bit problematic to implement such validators for custom containers like std::vector - for example validating that all entries are unique

[liuzicheng1987]

@vBazilevich sure...I never really thought that this is something people might want to do...