Add documentation regarding accessibility

[getsetgopi]

Want to know if chart.js is 100% accessibility compliance. Supports level A and AA as per WCAG 2 standards.

[benmccann]

You create the canvas element yourself and Chart.js populates it with content. When you create the canvas object you will need to do it in a way that is accessible. Two examples are below which are from http://pauljadam.com/demos/canvas.html:

Example 1:

<canvas id="chart" height="400" width="650" role="img" aria-label="Bar Chart Values in Millions from 0 to 12000. Human: 11000, Chimp: 6200, Dolphin: 5800, Cat: 300"> </canvas>

Example 2:

    <canvas id="can2" height="400" width="650">Text alternative for this canvas graphic is in the data table below.</canvas>
    <table border="0" cellpadding="5" summary="This is the text alternative for the canvas graphic.">
      <caption>
      Neurons in Cerebral Cortex
      </caption>
      <tbody>
        <tr>
          <th scope="col">Value</th>
          <th scope="col">Human</th>
          <th scope="col">Chimp</th>
          <th scope="col">Dolphin</th>
          <th scope="col">Cat</th>
        </tr>
        <tr>
          <th scope="row">Millions</th>
          <td>11000</td>
          <td>6200</td>
          <td>5800</td>
          <td>300</td>
        </tr>
      </tbody>
    </table>

We should add information about this to the documenation