docs(plugins): add css to panel example

examples/panel-boilerplate-es5/css/styles.css

@@ -0,0 +1,3 @@
+.panel-boilerplate-values {
+  text-align: center;
+}

examples/panel-boilerplate-es5/module.js

@@ -1,6 +1,7 @@
 define([
   'app/plugins/sdk',
-  'lodash'
+  'lodash',
+  './css/styles.css!'
 ], function(sdk, _) {
 
   var BoilerPlatePanelCtrl = (function(_super) {

examples/panel-boilerplate-es5/panel.html

@@ -2,6 +2,6 @@
 	Basic panel
 </h2>
 
-<p class="text-center">{{ctrl.values}}</p>
+<p class="panel-boilerplate-values">{{ctrl.values}}</p>