Add developer documentation as to why we add newlines at the end of each file
I'm not sure if this is already in our docs, if so, we can close this.
If not, we should add it because some developers may not realize why it's important as seen in https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/7545#note_41048286
Adding to the documentation could decrease back and forth about it's requirement