Elaborate getting a current working copy.

The `git checkout` step from #3098 got removed by some merge, so here it is again.
My point was that the assumption isn't necessarily justified, so making it even more explicit makes it worse.
If you're right and some people don't even know the `cd` command but weirdly still attempt to build an application from source, you `~/atom` will probably be the right folder as you say.

However, most people doing this are likely to have a special folder for their git repos or other projects into which they cloned this thing, rather than just dumping it all into their home folder.

Again, I'm just nitpicking here, rather pedantically. I'm sure it'll be just fine, because most people know what they're doing.