Add instructions for new breakpoints config

Author: ljwagerfield

README.md

@@ -223,6 +223,10 @@ const options = {
   showFinishButton: true,         // Show/hide the "finish" button in the widget.
   showRemoveButton: true,         // Show/hide the "remove" button next to each file.
   styles: {
+    breakpoints: {
+      fullScreenWidth: 750,       // Full-screen mode activates when the screen is at or below this width.
+      fullScreenHeight: 420       // Full-screen mode activates when the screen is at or below this height.
+    },
     colors: {
       primary: "#377dff",         // Primary buttons & links
       active: "#528fff",          // Primary buttons & links (hover). Inferred if undefined.