Add more content to <description> part of appdata

Summary:
Given the info in the appdata might be the first and only thing
especially newcomers to the world of (FLOSS)/linux software might see,
let's be more detailed here, to catch them when they are young ;)

Reviewers: KDevelop, apol, kfunk

Reviewed By: KDevelop, apol, kfunk

Subscribers: kfunk, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5350