add description

chrome/loading_indicator_bouncing_line.css

@@ -1,6 +1,8 @@
 /* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/loading_indicator_bouncing_line.css made available under Mozilla Public License v. 2.0
 See the above repository for updates as well as full license text. */
 
+/* Shows a left-right bouncing line at the top of the content area during navigation */
+
 @keyframes statusline-anim{from{background-position-x: left}to{background-position-x: right}}
 #statuspanel[type="status"]::after{
   position: fixed;