Document why tabindex is necessary when a11y: true

[julkue]

This would be helpful I think.

[callmecavs]

happy to look at a PR that adds this documentation to the a11y section of the README.

the reason is here on MDN. quoting that page:

A negative value (usually tabindex="-1") means that the element should be focusable, but should not be reachable via sequential keyboard navigation. Mostly useful to create accessible widgets with JavaScript.

more context from @Heydon on #16

[julkue]

I do understand the meaning of tabindex, but I don't understand why a section you've just scrolled to shouldn't be reachable by keyboard navigation?

[julkue]

However, when I have time I will read through the issue you've posted.

[callmecavs]

@julmot as I understand it, tabindex of -1 programmatically forces the focus. you can still apply a tabindex to the element - it'll just be overridden to force focus when that element is jumped to

@Heydon does a nice job of painting the full picture in that other issue