Extend README, add example models

README.md

@@ -8,18 +8,25 @@ In addition to the above formats, it also allows to open a ZIP file that contain
 
 There are two ways to open your files:
 
-1. Click on your file in an application like standard Android "Downloads" app. (Most other Android applications will also behave like that.)
+1. Click on any supported file in an application like standard Android _"Downloads"_ app. (Most other Android applications will also behave like that.)
 
-    This will pass the file to _view3dscene-mobile_ through a mechanism that *does not* reveal the actual filename of the original file.
+    This will pass the file to _view3dscene-mobile_.
 
-     This approach will only work for _self-contained_ model files. Such files don't refer to other files (like textures) through relative URLs. Examples that work:
+    An important limitation: This approach will only work for _self-contained_ model files. Such files cannot refer to other files (like textures) using relative URLs. The reason behind this is that _view3dscene-mobile_ doesn't get the file path from the Android system, so it cannot resolve relative URLs. We only get file contents.
 
+    Examples that work:
+
+    - glTF GLB variant.
+    - ZIP file that contains a model (X3D, glTF, ...) and the associated media (textures etc.)
+    - X3D/VRML with media embedded using [data URI scheme](https://en.wikipedia.org/wiki/Data_URI_scheme). You can use [to-data-uri](https://github.com/castle-engine/castle-engine/tree/master/tools/to-data-uri) utility distributed as part of [Castle Game Engine](https://castle-engine.io/) to convert any media to data URI.
+    - X3D/VRML with textures embedded using X3D/VRML `PixelTexture` node. Though we recommend "data URI", it is more universal.
     - Models that don't need any additional media (like textures).
     - Models that refer to the additional media using http/https links.
-    - ZIP file that contains a model (X3D, glTF, ...) and the associated media (textures etc.)
-    - X3D/VRML with media embedded using [data URI scheme](https://en.wikipedia.org/wiki/Data_URI_scheme). Specifically for textures, you can also embed them using X3D/VRML `PixelTexture` node.
-    - glTF GLB variant.
 
-2. Click on your file in an application like [Total Commander for Android](https://play.google.com/store/apps/details?id=com.ghisler.android.TotalCommander), that can pass to _view3dscene-mobile_ the actual filename.
+    Use the `example_models` directory for test models that work with this approach.
+
+2. Click on your file in an application like [Total Commander for Android](https://play.google.com/store/apps/details?id=com.ghisler.android.TotalCommander). This passes to _view3dscene-mobile_ the actual filename.
+
+    This works for all cases.
 
-    This is the perfect approach, that works in all cases.
+    Note that _view3dscene-mobile_ must have permissions to read from given external storage location. Make sure it is so, in the Android settings, under _Applications_ -> _view3dscene_ -> _Permissions_ -> _Storage_.