Document vulkan cmdline arg

Author: ardera

src/flutter-pi.c

@@ -94,6 +94,8 @@ OPTIONS:\n\
                              asset bundle directory.\n\
                              This also requires a libflutter_engine.so that was\n\
                              built with --runtime-mode=release.\n\
+\n\
+  --vulkan                   Use vulkan for rendering.\n\
 \n\
   -o, --orientation <orientation>  Start the app in this orientation. Valid\n\
                              for <orientation> are: portrait_up, landscape_left,\n\