README: Move to Markdown, rewrite introduction

Move the README file to Markdown, and update it to attempt to explain
the current status and use of Weston.

The first sections are user-facing, so they can quickly understand what
Weston is, what it does, what it doesn't do, and how to go about using
it. The following sections on libweston and for distribution packagers
are left intact, but should probably be moved to separate documents.

This includes a screenshot of Weston running weston-terminal, Chrome and
simple-egl, which was taken by myself and subject to the same licensing
terms as the rest of the tree.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>