Add notes to readme/documentation on accessibility

[jkjustjoshing]

Right now there's no note on how this library acts for a screen reader, keyboard user, or just a user that doesn't read very fast. These are important considerations for a UI component to consider.

Please check out https://www.w3.org/WAI/tutorials/carousels/ for more information on things to consider.

2 specific issues I noticed with a quick glance at the docs:

1. The buttons in the button example aren't buttons! Not friendly to keyboard OR screen reader users
2. The link to the keyboard navigation example from the README is broken. Keyboard navigation is an important thing for keyboard-only users, though make sure that the implementation follows W3 accessibility guidelines!

[amio]

@jkjustjoshing Thanks for pointing out! I'm planning a rewrite, it's a good opportunity to have accessibility support baked in from start.