Add example for constrained wheel zooming

examples/mousewheel-zoom.html

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+---
+layout: example.html
+title: Mousewheel/Trackpad Zoom
+shortdesc: Restrict wheel/trackpad zooming to integer zoom levels.
+docs: >
+  By default, the `ol.interaction.MouseWheelZoom` can leave the map at
+  fractional zoom levels. If instead you want to constrain wheel/trackpad
+  zooming to integer zoom levels, set <code>constrainResolution: true</code>
+  when constructing the interaction.
+tags: "trackpad, mousewheel, zoom, interaction"
+---
+<div id="map" class="map"></div>

examples/mousewheel-zoom.js

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+goog.require('ol.Map');
+goog.require('ol.View');
+goog.require('ol.interaction');
+goog.require('ol.interaction.MouseWheelZoom');
+goog.require('ol.layer.Tile');
+goog.require('ol.source.OSM');
+
+
+var map = new ol.Map({
+  interactions: ol.interaction.defaults({mouseWheelZoom: false}).extend([
+    new ol.interaction.MouseWheelZoom({
+      constrainResolution: true // force zooming to a integer zoom
+    })
+  ]),
+  layers: [
+    new ol.layer.Tile({
+      source: new ol.source.OSM()
+    })
+  ],
+  target: 'map',
+  view: new ol.View({
+    center: [0, 0],
+    zoom: 2
+  })
+});