Add the missing donut chart example

Added the missing donut chart example. This PR fixes #83

src/example/main.js

@@ -45,6 +45,7 @@ import TreemapExample from './treemap/dynamic-treemap';
 import StaticTable from './table/static-table';
 import DynamicTable from './table/dynamic-table';
 import SimpleRadialChart from './radial-chart/simple-radial-chart';
+import DonutChartExample from './radial-chart/donut-chart';
 import CustomRadiusRadialChart from './radial-chart/custom-radius-radial-chart';
 import VerticalDiscreteColorLegendExample from './legends/vertical-discrete-color';
 import HorizontalDiscreteColorLegendExample from './legends/horizontal-discrete-color';
@@ -153,7 +154,10 @@ const examples = (
         <h3>Simple Radial Chart</h3>
         <SimpleRadialChart />
       </section>
-
+      <section>
+        <h3>Simple Donut Chart</h3>
+        <DonutChartExample />
+      </section>
       <section>
         <h3>Custom Radius</h3>
         <CustomRadiusRadialChart />

src/example/radial-chart/donut-chart.js

@@ -0,0 +1,39 @@
+// Copyright (c) 2016 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+import React from 'react';
+import {RadialChart} from '../../';
+
+export default function DonutChartExample() {
+  return (
+    <RadialChart
+      innerRadius={100}
+      radius={140}
+      data={[
+          {angle: 2},
+          {angle: 6},
+          {angle: 2},
+          {angle: 3},
+          {angle: 1}
+        ]}
+      width={300}
+      height={300}/>
+  );
+}