Update readme to include information about changing body classes.

README.md

@@ -84,6 +84,19 @@ This theme includes a shortcode for a contact form that you can add to any page
 {{< form-contact action="http://formspree.io/your@email.com" >}}
 ```
 
+### Update font or body classes
+
+The theme is set by default to use a near-white background color and the "Avenir" or serif typeface. You can change these in your config file with the `body_classes` paramter, like this:
+
+```
+[params]
+  body_classes = "avenir bg-near-white"
+```
+
+You can find a list of available typefaces [here](https://github.com/tachyons-css/tachyons/blob/v4.7.0/src/_font-family.css).
+
+_n.b. in future versions we will likely separate the typeface and other body classes.
+
 
 ### Nearly finished