Elaborate getting a current working copy.

The `git checkout` step from #3098 got removed by some merge, so here it is again.
Perhaps a long-time Windows-only user currently tries to get comfortable with Linux and wants to use the latest features of his/her favorite editor? I think Atom should be prepared for that.

People who keep their clones somewhere else than their home directory, have most likely made that decision consciously. They can notice that the commands this document offers do not fit their file system policy and adapt them accordingly. Novice users can most likely live with a messy home directory or just delete the Atom build directory when they are done installing.

As an alternative to prefixing paths with `~/` one could also add an optional step "0." in which we advise the user to change the directory to where they keep their projects. (Why the hell isn't there a XDG-user-dir for projects?!) But that would make things easier for advanced users at the expense of novice users. I think that would be a rather poor decision.