docs: add contributing guide
Summary:
This adds a document providing information on how to contribute to KScreen.
It includes clarifications about the used coding style and introduces a Commit
Message Guideline based on the KDE Commit Policy and the Conventional Commits
specification.
Reviewers: Plasma
Subscribers: mart, davidedmundson, yurchor, broulik, ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24068