by bigbaldy: README.md update.

Includes information about sub-themes.

README.md

@@ -40,6 +40,10 @@ INSTALLATION
 Install as you would normally install a contributed Drupal module. Visit
 https://www.drupal.org/node/895232/ for further information.
 
+There is a sub-theme example that you can copy to your themes/custom
+directory. See the README.md file in the subtheme_starterkits direcory
+for additional information.
+
 CONFIGURATION
 -------------