README: Add basic info and .md prefix

Signed-off-by: Victor Toso <victortoso@redhat.com>
Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>

README.md

+spice-protocol
+==============
+
+The protocol definition for [SPICE] project used by:
+* [spice-server]: A server side library used by [QEMU] and [x11spice];
+* [spice-gtk]: A client side library used by [virt-viewer and virt-manager] and
+  [GNOME Boxes];
+* [linux/vd-agent] and [win32/vd-agent]: Guest components for Linux and Windows
+* [xf86-video-qxl], [gpu/drm/qxl], [win32/qxl], [win32/qxl-wddm-dod]: Guest video
+  drivers;
+
+Please [report bugs] at Freedesktop's instance of Gitlab
+
+[SPICE]: https://spice-space.org
+[spice-server]: https://gitlab.freedesktop.org/spice/spice
+[QEMU]: https://www.qemu.org
+[x11spice]: https://gitlab.com/spice/x11spice
+[spice-gtk]: https://gitlab.freedesktop.org/spice/spice-gtk
+[virt-viewer and virt-manager]: https://virt-manager.org/
+[GNOME Boxes]: https://wiki.gnome.org/Apps/Boxes
+[linux/vd-agent]: https://gitlab.freedesktop.org/spice/linux/vd_agent
+[win32/vd-agent]: https://gitlab.freedesktop.org/spice/win32/vd_agent
+[xf86-video-qxl]: https://gitlab.freedesktop.org/xorg/driver/xf86-video-qxl
+[gpu/drm/qxl]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/qxl
+[win32/qxl]: https://gitlab.freedesktop.org/spice/win32/qxl
+[win32/qxl-wddm-dod]: https://gitlab.freedesktop.org/spice/win32/qxl-wddm-dod
+[report bugs]: https://gitlab.freedesktop.org/spice/spice-protocol/issues/