[gnome-backgrounds] README: add minimal guidelines

[Jakub Steiner <jimmac src gnome org>]

commit c549822ae26a9db255371b5cba196b44be9a5be3
Author: Jakub Steiner <jimmac gmail com>
Date:   Thu Jun 11 09:35:10 2020 +0200

    README: add minimal guidelines

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-gnome-backgrounds
-=================
+# GNOME Backgrounds
 
 This module contains a set of backgrounds packaged with the GNOME desktop.
+
+## Design Guidelines
+
+![Defaults](defaults.jpg)
+
+### Default
+
+The default wallpaper has historically been strictly abstract, low contrast and provided in three daytime 
variants. The geometric pattern has oscillated around stripes and triangles and over the years has become 
part of the GNOME brand. Steering too much away from it will dillute the brand.
+
+### Supplemental
+
+It is desirable to have a wide mix of themes and colors for the additional wallpapers. Photographs and 
illustrations that would otherwise be too contrasty and with high frequency detail to be a good backdrop are 
perfectly suitable. Photos should generally be at least 3840x2160px, with no artifacts such as chromatic 
aberation. Generally 16:9 aspect ratio is preferred as the wallpapers get scaled with the `background-size: 
cover` equivalent method and wide displays are becoming more and more common.
+
+### Low fidelity and size constraints
+
+One wallpaper is reserved for plain color to be used for remote access where it's benefitial for the 
compression algorith to cope with it with ease. Unfortunately due to distribution limits, the background 
collection has to keep its size contrained to around 40MB. This forces us to replace assets rather than add 
them. You can download the [Nostalgia](https://flathub.org/apps/details/im.bernard.Nostalgia) app to view 
previous release wallpapers.
+
+## License
+
+CCBYSA / Public Domain / CC0 is preferred. All attribution goes to the AUTHORS file.
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