Update readme w/ command line information

README.md

@@ -4,3 +4,16 @@ Repository hosting source code for the [HDR + WCG Image Viewer UWP app](https://
 This app is based on code from the D2DAdvancedColorImages SDK sample which is part of the [Windows 10 samples repository](http://go.microsoft.com/fwlink/p/?LinkId=619979). It is the spiritual successor to the [HDR Image Viewer](https://www.microsoft.com/store/productId/9NPSWXVL7W40) published by Rick Manning.
 
 Releases/tags correspond to versions pushed to the Windows Store.
+
+## Command line usage
+You should invoke HDRImageViewer from the directory containing the image you wish to load - UWP apps launched from a command line only have access to files within the working directory.
+### Parameters
+`-f` Start in fullscreen mode
+
+`-h` Start with UI hidden
+
+`-input:[filename]` Load [filename]
+
+**Note: Filename must be relative to the current working directory as HDRImageViewer only has access to that directory.**
+### Example
+`HDRImageViewer.exe -f -h -input:myimage.jxr`