[libgnome-volume-control] README: Add README

[Bastien Nocera <hadess src gnome org>]

commit 172ccc40bb02b4190ead945b628aa3de9414f0a0
Author: Bastien Nocera <hadess hadess net>
Date:   Fri Sep 7 15:31:14 2018 +0200

    README: Add README
    
    Closes: #1

 README.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)
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+# libgnome-volume-control
+
+libgnome-volume-control is a copy library that's supposed to be used as
+a git sub-module. If your project uses some of libgnome-volume-control's
+strings in a user-facing manner, don't forget to add those files to your
+POTFILES.in for translation.
+
+## Projects using libgnome-volume-control
+
+- [gnome-shell](https://gitlab.gnome.org/GNOME/gnome-shell)
+- [gnome-settings-daemon](https://gitlab.gnome.org/GNOME/gnome-settings-daemon)
+- [gnome-control-center](https://gitlab.gnome.org/GNOME/gnome-control-center)