[gnome-desktop] Add CONTRIBUTING.md to discourage bug reports

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 870618416465c2c5b2690185ff3dbf3d38e1b0a1
Author: Michael Catanzaro <mcatanzaro posteo net>
Date:   Thu Dec 27 20:26:34 2018 +0000

    Add CONTRIBUTING.md to discourage bug reports

 CONTRIBUTING.md | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 00000000..5c37c79e
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,5 @@
+Please don't report issues against gnome-desktop unless you really know what
+you're doing. This repository contains the libgnome-desktop library. It's not
+the right place to report bugs affecting GNOME's desktop.
+
+[Report desktop bugs here instead.](https://gitlab.gnome.org/GNOME/gnome-shell/issues)
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