update headless example README

examples/dart/cli_headless/README.md

@@ -1,3 +1,9 @@
 # cli_headless
 
-An example of Thermion running via Dart CLI with no hardware accelerated rendering surface.
+A simple headless Dart CLI app using Thermion to render to a bitmap.
+
+To run:
+`dart --enable-experiment=native-assets run bin/example.dart`
+
+Currently, this will probably only work  on a non-virtualized macOS host. 
+