Updated README regarding the scrolling background.

Author: popper73

README.md

@@ -21,7 +21,36 @@ Copy the config.toml file from the exampleSite folder into your root folder of y
 
 ### Change the background
 
-Should you choose to change the background, replace the `bg.jpg` file in `themes/aerial/static/css/images`.
+Should you choose to change the background, create a `/static/css/images` folder and add a picture named `bg.jpg`. For the best results, follow these instructions:
+
+The Scrolling Background:
+
+    This relies entirely on CSS to do its thing, which is cool, but that makes
+    changing it a bit weird/tricky at first. You can still use pretty much any image
+    you want, but for best results make sure yours is:
+
+    - Horizontally tileable.
+    - Wide and short.
+    - About 1500px wide.
+    - Fades to a solid color either at the top of bottom (which is used to fill
+      the empty space above or below your image).
+
+    Now, there are two ways to use it: with CSS, or with Sass:
+
+    CSS:
+
+        Look for this line in css/style.css (line 108 as of this writing):
+
+            background: #348cb2 url("images/bg.jpg") bottom left;
+
+        and use it to set the page background color, URL, and placement of
+        your image. It should be as close to 1500px wide as you can get it.
+
+    Sass:
+
+        Set the value of $bg to the page background color, URL, and placement
+        of your image. Change $bg-width if your image is something other than
+        1500px wide.
 
 ## License