Added code example for setenv class

Example usage is based on the Apache docs' own example for the SetEnv directive: 
http://httpd.apache.org/docs/2.2/mod/mod_env.html#setenv

Change requested here:
https://tickets.puppetlabs.com/browse/DOCUMENT-204

README.md

@@ -1426,6 +1426,14 @@ Sets the servername corresponding to the hostname you connect to the virtual hos
 
 Used by HTTPD to set environment variables for vhosts. Defaults to '[]'.
 
+Example:
+
+```puppet
+     apache::vhost {'setenv.example.com':
+         setenv  => ['SPECIAL_PATH /foo/bin'],
+      }
+```
+
 #####`setenvif`
 
 Used by HTTPD to conditionally set environment variables for vhosts. Defaults to '[]'.