https://www.mediawiki.org/wiki/Extension:Newsletter looks like a pretty decent extension page, but
we have worked a lot on that extension after the bulk of that page was written. It needs verification. Is the information in that page up to date? Is there key information that is missing?
In order to complete this task properly, you should have a clean MediaWiki 1.27+ installation and proceed to install the extension following the instructions in that page. Then you should check that any pages/features described are correct.
Note that the audience for that page are wiki administrators. Help documentation will be found in another page (see T154222: Create Newsletter Extension Help page).
Although I can help proxy-mentoring this task for Google-Code-In-2016, I would feel more comfortable if @01tonythomas is the person who actually evaluates the task, since he knows the tech involved, dependencies, etc.