Add React upgrade to changelog

Author: japgolly

doc/changelog/1.0.0-RC.md

@@ -68,6 +68,7 @@ See https://github.com/japgolly/scalajs-react/blob/v1.0.0-RC1/doc/changelog/1.0.
 * Add `Reusable.ap(ra: Reusable[A], rb: Reusable[B])(f: (A, B) => C): Reusable[C]`
 * Add `Reusable[A]#ap[B](rf: Reusable[A => B]): Reusable[B]`
 * Add `Reusable[A]#tuple[B](rb: Reusable[B]): Reusable[(A, B)]`
+* Upgrade to React to 15.5.4.
 * Upgrade to Scala.JS 0.6.15.
 * Upgrade to Scala to 2.11.11 and 2.12.2.
 * Upgrade to Scalaz 7.2.11.