Add developer documentation as to why we add newlines at the end of each file

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

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 gitlab-com/www-gitlab-com!7545 (comment 41048286)

Adding to the documentation could decrease back and forth about it's requirement