README.md: Add more/clean up links to consuming projects
README.md: Add more/clean up links to consuming projects

Add meta-updater and QtOTA, and delete OpenEmbedded since it's implied by the
first two. Merge rpm-ostree + Atomic Host since they're close. Clarify
gnome-continuous a bit.

Closes: #700
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree