Updated after feedback, incorporated references to clarify usage of s…

…imulacrum in cats kernel

docs/src/main/tut/faq.md

@@ -38,7 +38,9 @@ It's really common to have a `List` of values with types like `Option`, `Xor`, o
 
 ## <a id="simulacrum" href="#simulacrum"></a>What does `@typeclass` mean?
 
-Cats defines and implements numerous type classes. Unfortunately, encoding these type classes in Scala can incur a large amount of boilerplate. To address this, [Simulacrum](https://github.com/mpilquist/simulacrum) introduces `@typeclass`, a macro annotation which generates a lot of this boilerplate. This elevates type classes to a first class construct and increases the legibility and maintainability of the code. Use of Simulacrum also ensures consistency in how the type classes are encoded across a project. Cats uses Simulacrum wherever possible to encode type classes, and you can read more about it at the [project page](https://github.com/mpilquist/simulacrum).
+Cats defines and implements numerous type classes. Unfortunately, encoding these type classes in Scala can incur a large amount of boilerplate. To address this, [Simulacrum](https://github.com/mpilquist/simulacrum) introduces `@typeclass`, a macro annotation which generates a lot of this boilerplate. This elevates type classes to a first class construct and increases the legibility and maintainability of the code. Use of simulacrum also ensures consistency in how the type classes are encoded across a project. Cats uses simulacrum wherever possible to encode type classes, and you can read more about it at the [project page](https://github.com/mpilquist/simulacrum).
+
+Note that the one area where simulacrum is intentionally not used is in the `cats-kernel` module. The `cats-kernel` module is intended to be a shared dependency for a number of projects, and as such, it is important that it is both lightweight and very stable from a binary compatibility perspective. At some point there may be a transition from simulacrum to [typeclassic](https://github.com/typelevel/typeclassic), and the binary compatibility of moving between simulacrum and typeclassic is unclear at this point. Avoiding the dependency on simulacrum in `cats-kernel`, provides insulation against any potential binary compatibility problems in such a transition.
 
 ## <a id="kind-projector" href="#kind-projector"></a>What do types like `?` and `λ` mean?