Add overviewmap example, simple use case

examples/overviewmap.js

@@ -0,0 +1,22 @@
+goog.require('ol.Map');
+goog.require('ol.View');
+goog.require('ol.control');
+goog.require('ol.control.OverviewMap');
+goog.require('ol.layer.Tile');
+goog.require('ol.source.OSM');
+
+var map = new ol.Map({
+  controls: ol.control.defaults().extend([
+    new ol.control.OverviewMap()
+  ]),
+  layers: [
+    new ol.layer.Tile({
+      source: new ol.source.OSM()
+    })
+  ],
+  target: 'map',
+  view: new ol.View({
+    center: [500000, 6000000],
+    zoom: 7
+  })
+});