Guideline for adding functionality without breaking binary compatibility.

[kailuowang]

To maintain binary compatibility we can't add new methods with default implementation to existing traits. However we can add new traits with those and have classes extending them.
For example, we want to add a new method to a Typeclass TC[F[_]]. we could either provide a syntax trait that provides that method like

trait BOps2 {
    abstract class BOps2Ex[F[_], A](fa: F[A])(implicit val F: TC[F]) {
       def newMethod ???
    }
   implicit def bops[F[_], A](fa: F[A])(implicit val F: TC[F]) = new BOps2Ex(fa) 
}  

If this new method cannot be provided as a syntax to the data types we can add an implicit class against the Typeclass itself.
In both case, we can't have any existing trait extend these new trait. We would need to have several objects in cats inherit this new trait explicitly including cats.implicits cats.syntax.all, etc.

We should decide on a strategy and document in guideline.

[kailuowang]

#2148 provides a fine example.