Update readme

Author: drhelius

README.md

@@ -133,6 +133,7 @@ Don't hesitate to report bugs or ask for new features by [opening an issue](http
 ### Debugging Features
 - **Docking Windows**: In debug mode, you can dock windows together by pressing SHIFT and dragging a window onto another.
 - **Multi-viewport**: In Windows or macOS, you can enable "multi-viewport" in the debug menu. You must restart the emulator for the change to take effect. Once enabled, you can drag debugger windows outside the main window.
+- **Single Instance**: You can enable "Single Instance" in the ```Emulator``` menu. When enabled, opening a ROM while another instance is running will send the ROM to the running instance instead of starting a new one.
 - **Debug Symbols**: The emulator automatically tries to load a symbol file when loading a ROM. For example, for ```path_to_rom_file.rom``` it tries to load ```path_to_rom_file.sym```, ```path_to_rom_file.lbl``` and ```path_to_rom_file.noi```. You can also load symbol files using the GUI or the CLI. It supports *cc65* (VICE label file), *lyxass* (EQU) and *mads* (lab and hea) file formats.
 
 ### Command Line Usage