Readme Needs Minor Updates

[CodingFlow]

Readme needs a few adjustments:

• The example code shows usage of [Range(18, 99)] attribute, but there is no such attribute provided by Validly Extensions package.
• Clarify that only validation attributes recognized by Validly are the ones from the Validly Extensions package and from the Validly.Extensions.Validators namespace. I did not know this, and tried to use the Range attribute and Visual Studio suggested to use the one from System.ComponentModel.DataAnnotations, which I accepted. This led me to believe that I could use the attributes from that namespace. Which appears to not be the case.

[Hookyns]

Ad 1) Fixed. It is Between, as stated in the documentation.
Ad 2) I will add a note about DataAnnotations attributes. To clarify: Validly recognizes all attributes marked with the Validator attribute, not only the predefined validators provided by Validly.