Add comment about where newtab-image is loaded from

content/newtab_background_image.css

@@ -1,9 +1,10 @@
 /* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/content/newtab_background_image.css made available under Mozilla Public License v. 2.0
 See the above repository for updates as well as full license text. */
 
-@-moz-document url(about:newtab), url(about:home){
+@-moz-document url("about:newtab"), url("about:home"){
   body{
-    background-image: url("image.jpg");
+    /* This will load image.jpg stored in the same folder as this file */
+    background-image: url("image.jpg"); 
     background-size: cover;
     background-repeat: no-repeat;
     background-attachment: fixed;