[mutter] Add README.md

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit eb0865fe11ee70a43754645aff1b654fce1b4466
Author: Jonas Ådahl <jadahl gmail com>
Date:   Mon Jul 30 20:55:31 2018 +0200

    Add README.md
    
    The welcome page on Gitlab is pretty boring for mutter. Improve that a
    bit by adding a README file with some basic introduction.
    
    Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/245

 README.md | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
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diff --git a/README.md b/README.md
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+# Mutter
+
+Mutter is a Wayland display server and X11 window manager and compositor library.
+
+When used as a Wayland display server, it runs on top of KMS and libinput. It
+implements the compositor side of the Wayland core protocol as well as various
+protocol extensions. It also has functionality related to running X11
+applications using Xwayland.
+
+When used on top of Xorg it acts as a X11 window manager and compositing manager.
+
+It contains functionality related to, among other things, window management,
+window compositing, focus tracking, workspace management, keybindings and
+monitor configuration.
+
+Internally it uses a fork of Cogl, a hardware acceleration abstraction library
+used to simplify usage of OpenGL pipelines, as well as a fork af Clutter, a
+scene graph and user interface toolkit.
+
+Mutter is used by GNOME Shell, the GNOME core user interface. It can also be run
+standalone, using the command "mutter", but just running plain mutter is only
+intended for debugging purposes.
+
+## License
+
+Mutter is distributed under the terms of the GNU General Public License,
+version 2 or later. See the [COPYING][license] file for detalis.
+
+[bug-tracker]: https://gitlab.gnome.org/GNOME/mutter/issues
+[license]: COPYING