contrib/model_viewer: update README file

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>

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 Sample portable 3D app implemented with the gamnit depth framework
 
+This application uses the _depth_ framework to load and display 3D models.
+It also applies the _flat_ framework to display the static UI elements (like the "Next model" label).
+
+The pretty earth model (`GlobeModel`) is implemented by a material with a custom graphical program.
+It renders the earth with an displaced surface, a cloud layer, city lights and Phong lighting effects on the water.
+
+# Variations
+
+* The standard application is compiled to a desktop executable at `bin/model_viewer` and an Android package at `bin/model_viewer.apk`.
+* The virtual reality variant `bin/model_viewer_vr.apk` targets Android and uses Google Cardboard for head tracking.
+
 # Art
 
 * 3D models `Tree_01` and `Oak_Fall_01` were created by Kenney.nl and published under CC0.
 * 3D model `Quandtum_BA-2_v1_1` was created by Quandtum and published under CC0.
 * Some textures on the `Quandtum_BA-2_v1_1` model have been created with images from goodtextures.com.
   These images can be used in free softwares, some restrictions still applies for modifications and other uses.
+* Globe textures credit: NASA, Visible Earth