Add docs website to the "About" section on github

[vituri]

The "About" section is the most usual place to put the docs website. I had to find the link on the "Deployments" section.

http://juliastats.org/Clustering.jl/

[alyst]

We have documentation badges in the README.md -- which is the conventional location of the documentation links for Julia projects.
Do you mean something else by the "About" section?

[vituri]

Ops, sorry if I was not clear. I mean this section:

Usually almost all packages I see have the docs on this part of the github page.

[alyst]

Ah, now I see! Thanks, I have added the link.