Update doc to show 'label inside (/middle of) doughnut' chart recipe

Example code can be found at https://codepen.io/anon/pen/xqMQQB?editors=1010

and

https://stackoverflow.com/a/43176415/1410291

It is better, if this info is reflected in README as it is super helpful. Also, thanks for this amazing react wrapper over chartjs-2

Including code sample, in case one of those source links gets broken in future

import React from 'react';
import ReactDOM from 'react-dom';
import {Doughnut} from 'react-chartjs-2';

// some of this code is a variation on https://jsfiddle.net/cmyker/u6rr5moq/
var originalDoughnutDraw = Chart.controllers.doughnut.prototype.draw;
Chart.helpers.extend(Chart.controllers.doughnut.prototype, {
  draw: function() {
    originalDoughnutDraw.apply(this, arguments);
    
    var chart = this.chart;
    var width = chart.chart.width,
        height = chart.chart.height,
        ctx = chart.chart.ctx;

    var fontSize = (height / 114).toFixed(2);
    ctx.font = fontSize + "em sans-serif";
    ctx.textBaseline = "middle";

    var sum = 0;
    for (var i = 0; i < chart.config.data.datasets[0].data.length; i++) {
      sum += chart.config.data.datasets[0].data[i];
    }

    var text = sum,
        textX = Math.round((width - ctx.measureText(text).width) / 2),
        textY = height / 2;

    ctx.fillText(text, textX, textY);
  }
});

const data = {
	labels: [
		'Red',
		'Green',
		'Yellow'
	],
	datasets: [{
		data: [300, 50, 100],
		backgroundColor: [
		'#FF6384',
		'#36A2EB',
		'#FFCE56'
		],
		hoverBackgroundColor: [
		'#FF6384',
		'#36A2EB',
		'#FFCE56'
		]
	}]
};

class DonutWithText extends React.Component {

  render() {
    return (
      <div>
        <h2>React Doughnut with Totalled Data</h2>
        <Doughnut data={data} />
      </div>
    );
  }
};

ReactDOM.render(
  <DonutWithText />,
  document.getElementById('root')
);

[jerairrest]

@xPhantomNL I had it implemented on the gh pages but ran into an issue where a Chart applys plugins to all the charts in the app. (Totally a different bug)

To fix this example change,

import {Doughnut} from 'react-chartjs-2';

to

import {Doughnut, Chart} from 'react-chartjs-2';

[walkingalchemy]

@jerairrest Any chance you’ve made progress on plugins applying to all charts bug? I ran into it this morning and it seems to shut out the entire possibility of using plugins in an app that has more than one chart.