Add screen shot to README

A screen shot in README is good for people who encounter the project for
the first time as it immediately tells what to expect from the final
results -- people often look at a screen shot even before they read
anything.  In this particular case it also clearly shows that the
project utilizes Graphite's graphs and not its own (as
[Graphene](http://jondot.github.com/graphene/) does, for example).

closes #31

README.md

@@ -6,8 +6,9 @@ A simple Graphite dashboard built using Twitter's Bootstrap.
 Adding new dashboards is very easy and individual graphs are
 described using a small DSL.
 
-See the _sample_ directory for an example dashboard including
-a screenshot.
+See the _sample_ directory for a sample dashboard configuration.
+
+![Sample dashboard](https://github.com/ripienaar/gdash/raw/master/sample/email.png)
 
 Config?
 -------