cleanup litter in main dir

Summary:
- drop ChangeLog; not curated anymore
- drop INSTALL; out of date and there is better documentation online for
  building kf5 software
- NEWS; empty
- README; drop outdated stuff and convert to markdown so it gets rendered
  in repo viewers and is easier on the eyes when viewed in plaintext

(Leaving TODO behind even though it is not cureated it still has useful
 ideas for future reference)

Reviewers: sandsmark

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D8937