README: clarify how to start working on this site locally#1890

dscho merged 3 commits intogit:gh-pagesgit/git-scm.com:gh-pagesfrom

dscho:explain-better-how-to-develop-this-site-with-sparse-checkoutsdscho/git-scm.com:explain-better-how-to-develop-this-site-with-sparse-checkoutsCopy head branch name to clipboard