Elaborate getting a current working copy.

The `git checkout` step from #3098 got removed by some merge, so here it is again.
Ok I kind of figured that. In that case the `cd atom` of the second section is rather random, assuming that the person is always one level up from the cloned repo when they do this.
Of course people aren't idiots, but for coherence's sake I'm just wondering if it's better to take it out and instead a sentence like "Change into the folder of your cloned atom repository." 

Just a thought.