Release notes

• Added format support for message placeholders.
• Added support for overriding indexers for collection rules.
• Allow ErrorCode to be used to override localization key in the Language Manager.
• Additional error details are now shown in the ShouldNotHaveValidationErrorFor test extension.
• Conditions defined with root-level When/WhenAsync/Unless/UnlessAsync are now only executed once (instead of once for each rule).
• Support Otherwise method chained from root-level When/WhenAsync/Unless/UnlessAsync.
• Minor wording changes to default English error messages.
• Added Skip property to MVC5's CustomizeValidatorAttribute to bring parity with AspNetCore and WebApi integration.
• Japanese translation of the default error messages.
• Remove type constraint on the OverridePropertyName overload that takes an expression.