made the readme files more explicit.

[Jayaditya-Contributor]

(I have made the changes at starter part of the readme)

Before contribution :-

$ git clone git@github.com:yarnpkg/website.git yarn-website
$ cd yarn-website

After contribution :-

 Fork the repo using the fork button at the top right.
$ git clone git@github.com:yarnpkg/website.git yarn-website
$ cd yarn-website

[Haroenv]

I don't think this instruction is needed, as it would be the same for every repository. Having the name of the repo linked in the docs makes sense however.