Add README files to 3D demos

3d/material_testers/README.md

@@ -0,0 +1,17 @@
+# Material Testers
+
+This demo includes many sphere-like objects with complex materials,
+for the purpose of showcasing Godot's rendering capabilities.
+
+This demo was featured at the beginning of the
+[Godot 3.0 trailer](https://www.youtube.com/watch?v=XptlVErsL-o).
+
+Language: GDScript
+
+Renderer: GLES 3
+
+## Screenshots
+
+![Screenshot](screenshots/stones.png)
+
+![Screenshot](screenshots/mirror.png)

3d/material_testers/project.godot

@@ -15,7 +15,7 @@ _global_script_class_icons={
 
 [application]
 
-config/name="Material Tester"
+config/name="Material Testers"
 run/main_scene="res://material_tester.tscn"
 config/icon="res://icon.png"
 
@@ -36,4 +36,3 @@ multithread/thread_rid_pool_prealloc=60
 
 vram_compression/import_etc2=false
 quality/filters/msaa=2
-environment/default_environment="res://default_env.tres"