Improve package documentation

Summary:
The old readme file was just 4 outdated lines on how to compile the package..

I've now tried to add all the crucial info and write the file in Markdown.

CC-BUG: 414289

Test Plan:

Reviewers: Plasma, VDG, ngraham, davidedmundson

Reviewed By: Plasma, VDG, ngraham, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25415