Fixes copy releases folder

Hi Jesse,
first of all, thank you for this example, it is very helpful. As you can see I made two changes, one in the readme because in R17.1, there are some changes in releases versions (p.e.: erts, from 6.0 to 6.1), so it is needed to execute 'fix-slim-release'. The second change is in the fix-slim-release script, because I think it worked fine in past versions of Nitrogen, but now that the project is generated outside nitrogen folder, it can't mv the releases folder from /tmp.

Tell me what you think about this change, because I made it in my nitrogen fork too, but I prefer you have a look to it before making the pull request.

Thanks.