Guideline for adding functionality without breaking binary compatibility.

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.

Start by reviewing the binary-compatibility examples in the issue and the entry points it names: cats.implicits and cats.syntax.all. Decide and document a consistent strategy for adding methods through new syntax traits or implicit classes, including which existing traits and objects must not extend them. Done means the guideline records the agreed approach and examples clearly enough to apply to future changes.