Add documentation link to Hackage docs / description.

[stephen-smith]

int_index (on reddit):

The introduction covers them: https://int-index.github.io/ether/

I should probably add this link to the package description.

Creating an issue so it doesn't get forgotten.

[int-index]

I believe I did that a few minutes after posting the comment. Perhaps your browser has cached the Hackage page? Please check again.

[int-index]

Closing because the link is certainly in the description: