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