Additional info in the readme on how to use the tool without JSON files.

Author: mcourteaux

README.md

@@ -39,6 +39,20 @@ git submodule update
 
 ## CLI interface
 
+This program has two ways of being used:
+ - through Blender-style JSON configuration files.
+ - by manually specifying the lens characteristics on the command line.
+
+Originally, this tool was made to convert images generated by Blender, from one
+lens type to another. However, as this tool had potential to be used in other
+contexts, where no configuration files from Blender were available, simpliy
+specifying which lens-type the input image has directly on the command-line
+was very desirable. To specify the input image lens-type directly on the
+command-line, use the `--no-configs` argument along with `--i-...` flags under
+the `Input optics` section. When using `equirectangular` panoraminc input
+images, the `--rotation` flag might prove useful.
+
+
 ```
 Usage:
   ./bin/reproject [OPTION...]