Add default dashboards for StackLight

This patch adds the possibility to add StackLight dashboards when
installing grafana. By default they are not installed.
diff --git a/README.rst b/README.rst
index 1aae1e1..26daa01 100644
--- a/README.rst
+++ b/README.rst
@@ -44,6 +44,20 @@
           user: grafana
           password: passwd
 
+Server installed with default StackLight JSON dashboards
+
+.. code-block:: yaml
+
+    grafana:
+      server:
+        enabled: true
+        admin:
+          user: admin
+          password: passwd
+        dashboards:
+          enabled: true
+          path: /var/lib/grafana/dashboards
+
 
 Collector setup
 ---------------