Introduce coding-conventions.md

Summary:
Hopefully, the new document makes it clear when one can use QRect::right()
and QRect::bottom().

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27060