docs: readme: add missing --workdir metadata variable

| --version          | Show the current version                                |

| --delay-init SECS  | Delay the initial action execution (in s, default: 1.0) |

| --delay-press SECS | Delay the press actions execution (in s, default: 0.5)  |

| --workdir          | Use a specific working directory path                   |

| --workdir WORKDIR  | Use a specific working directory path                   |

| --up               | Press the <UP> key                                |

| --down             | Press the <DOWN> key                              |

| --left             | Press the <LEFT> key                              |