Example for value classes in docs

[mtsokol]

Hi,
I've recently tried to handle value classes in circe without nested structure.
Only found issue #469 which lacks in full example.
I've figured it out, but as circe beginner I wish I found it in docs (e.g. in semi-automatic-derivation).

Would you mind accepting such docs pull request?

import io.circe.Encoder
import io.circe.syntax._
import io.circe.generic.extras.semiauto._
import io.circe.generic.extras.defaults._

implicit val UserEncoder: Encoder[User]         = deriveEncoder
implicit val UserIdEncoder: Encoder[UserId]     = deriveUnwrappedEncoder
implicit val UserNameEncoder: Encoder[UserName] = deriveUnwrappedEncoder

case class UserId(id: Int) extends AnyVal
case class UserName(name: String) extends AnyVal
case class User(userId: UserId, userName: UserName)

val usr = User(UserId(1446), UserName("User Name"))

usr.asJson
//  res0: io.circe.Json =
//  {
//    "userId" : 1446,
//    "userName" : "User Name"
//  }

[captainju]

Same for the decoder counterpart.

[GMadorell]

Thanks for putting this issue out there! For me it was the only way to find it in google 👍 .
Also, personally I would like for being able to automatically use this as the default option (maybe by using a different import?) for automatically derivating the codecs. In the project I'm currently working on 100% of the encoders and decoders are like this.