Add the docs URL at the top on GitHub

[ocefpaf]

@efiring I don't have the rights here to do that but it would be nice to have the link https://TEOS-10.github.io/GSW-Python at the to like most projects do.

Here is a screeshot of it using my fork:

PS: it helps some automated tools to find the docs too.

[efiring]

Good idea, thank you. Done.

[ocefpaf]

Thanks!