Add note about Reusability/Reusable being in core now

Author: japgolly

doc/PERFORMANCE.md

@@ -9,6 +9,9 @@ These utilities help you avoid work in two ways.
 1. By making components' `shouldComponentUpdate` fns both easy to create, and accurate (safe). If it compiles, the logic in `shouldComponentUpdate` will be what you expect.
 2. By allowing you to cache your own arbitrary data, and build on it in a way such that derivative data is also cached efficiently.
 
+`Reusability`/`Reusable` is part of `core` (as of v1.4.0) where as
+the other utilities are part of the `extra` module.
+
 ```scala
 libraryDependencies += "com.github.japgolly.scalajs-react" %%% "extra" % "1.3.1"
 ```