Elaborate getting a current working copy.

The `git checkout` step from #3098 got removed by some merge, so here it is again.
I think it would be best to give the users both an instruction in natural language and bash where possible. In this case the user might be unfamiliar with bash and does not know how to change the directory, but will still get the job done by copying and pasting the command.

When opening a terminal one is usually in the home directory and that is implicitly assumed here. (The user will usually not have write access to say `/` for example and could therefore not build there.) Perhaps we could just make that implicit assumption explicit by prefixing paths with `~/` like this:

    cd ~/atom