Document that Scala.JS must be upgraded to 0.6.15

Author: japgolly

doc/changelog/1.0.0.md

@@ -383,6 +383,7 @@ then copy-paste it into your shell as a single command.
 
 These are the breaking changes that must be manually updated:
 
+* Upgrade Scala.JS to 0.6.15
 * If you're using any of these libraries, upgrade them to:
   * Monocle 1.4.0
   * ScalaCSS 0.5.3