Created Command line usage (markdown)

Command-line-usage.md

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+Launch vimix from command line:
+
+Usage: 
+
+      vimix [-H, --help] [-V, --version] [-F, --fontsize] [-L, --headless] [-S, --settings] [-T, --test] [-C, --clean] [filename]     
+
+ 
+Options:            
+  `--help`       : Display usage information      
+  `--version`    : Display version information      
+  `--fontsize`   : Force rendering font size to specified value, e.g., '-F 25'      
+  `--settings`   : Run with given settings file, e.g., '-S settingsfile.xml'      
+  `--headless`   : Run without GUI (only if output windows configured)      
+  `--test`       : Run rendering test and return      
+  `--clean`      : Reset user settings   
+
+Filename:      
+  vimix session file (.mix extension)      
+
+
+## `--settings`
+
+Specifies an XML settings file to load at runtime. e.g.,
+
+    vimix --settings /home/username/settingsvimix.xml
+
+XML settings files are exported from the settings panel:
+
+![image](https://github.com/brunoherbelin/vimix/assets/60393189/a86c583e-6f9d-4b59-b5c9-d4188d930fb7)
+
+
+## `--headless`
+
+Runs *vimix* without the main graphical user interface, showing only the output window(s).     
+Vimix still accepts OSC messages and user inputs from input mapping (keyboard, joysick, etc.).   
+
+    vimix --headless
+
+NB: if the settings configuration has no output window, then the GUI is launched anyway.
+
+In combination with `--settings`, this allows running *vimix* in a standalone mode to show the output window(s) executing a session without the needs for GUI controls, e.g.,
+
+    vimix --headless --settings /home/username/settingsvimix.xml
+
+## `--fontsize`
+
+Enforces to use the specified font size for rendering the user interface. e.g., 
+
+    vimix --fontsize 40
+
+Font size is normally automatically defined based on the rendering resolution of the monitor at runtime. It usually ranges between 10 and 30 pixels in  height. This options is useful in some cases where the system cannot detect the monitor resolution.
+
+
+## `--clean`
+
+Cleans up all settings and exit. 
+
+    vimix --clean
+
+Settings are automatically stored in user's local settings folder (`/home/[username]/.config/vimix/` folder under linux, `/Users/[username]/Library/Application Support/vimix` folder under Mac OSX). The clean option allows resetting all settings in case a configuration problem occurs.
+
+## `--test`
+
+Performs a display test and exits with success or failure command code
+
+    vimix --test
+
+Should respond 'test OK` with return code 1. This is useful to validate installation status.
+
+
+
+