Add docs about dynamic validators

[franleplant]

After talking to one of our first users we came up with a use case where he was using minNumber but he needed that the argument to be bound to another field. This was the best solution we came up with:

validationRules = {
  priceMax: {
    minNumber: value => validators.get('minNumber')(value, this.state.fields.priceMin)
  }
}

The idea is to document this case in the readme. The solution is basically that.
We use the already existing validator but we use it dynamically by overwriting it with it's own function.