For easier navigation of changes, this introduces `NEWS.md`, a high-level
overview of important or otherwise noteworthy changes. It is a short list only,
with just the bare minimum of details. The new `UPGRADING.md` on the other hand,
is a much more detailed document, and contains the upgrade notes from plugins
too. The plugin docs now document the current version of the plugins only.
The intent is to have both a short overview (`NEWS.md`), and a detailed
guide (`UPGRADING.md`). The former will contain historical entries too, while
the later is meant to be an upgrade guide from the previous version to the most
recent one, with old notes eventually removed.
Fixes#436.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>