Update option docs for --externals (default peerDependencies & dependencies) (#636)

Author: gtrufitt

README.md

@@ -213,7 +213,7 @@ Options
 	-f, --format     Only build specified formats (any of modern,es,cjs,umd or iife) (default modern,es,cjs,umd)
 	-w, --watch      Rebuilds on any change  (default false)
 	--target         Specify your target environment (node or web)  (default web)
-	--external       Specify external dependencies, or 'none'
+	--external       Specify external dependencies, or 'none' (default peerDependencies and dependencies in package.json)
 	--globals        Specify globals dependencies, or 'none'
 	--define         Replace constants with hard-coded values
 	--alias          Map imports to different modules