Add a new markdown README

Summary:
The old readme was targetted at application developers having
conceptional questions about window management. While we all know that
this is very often needed, I doubt that any application developer is
looking at KWin's README to figure out why their window doesn't get
focus.

The new README is focussing more on potential new contributors by
describing what KWin is and how to reach our team. E.g as a nice small
document readable on git project hosting sides.

The information in the readme is mostly taken from
https://community.kde.org/KWin/Mission_Statement

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17722