steamplay-custom/compatibilitytool.vdf

"compatibilitytools"
{
	"compat_tools"
	{
		"custom-tool" // Internal name of this tool
		{
			"install_path" "."
			"display_name" "SteamPlay Custom"
			"from_oslist"  "windows"
			"to_oslist"    "linux"
		}
	}
}