Tweak README-KeyTab and README.keyboard a bit
Summary:
Rename README.KeyTab to README-KeyTab to prevent Konsole from trying to
load it as a keyboard translator file, and fix references to that file
name in other files.
Flesh out README-KeyTab and README.keyboard and reformat them.
Also copy the the original README-KeyTab to README-KeyTab-original,
just in case there's some concept that I misinterpretted/misunderstood.
Reviewers: Konsole, hindenburg
Reviewed By: Konsole, hindenburg
Subscribers: Konsole
Tags: Konsole
Differential Revision: https://phabricator.kde.org/D12032