Add guides for ARIA usage

[lwc]

Aria tags are important for web accessibility, yet this site doesn't guide about them.

Do note in certain countries not conforming websites for web accessibility is actually illegal.

For example, aria-describedby="id" allows blind people to get a description of a certain image:

<img src="w3schools_logo.png" alt="W3Schools logo" aria-describedby="image_desc">
<p id="image_desc">This image is the official logo of w3schools.com. It is shaped like...</p>

Some relevant resources:

1. https://www.w3.org/TR/WCAG20-TECHS/wai-aria_notes.html
2. https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques

Edited August 25, 2017 by lwc