Missing prompt documentation

[Liupold]

Prompt documentation must be added before the 0.7 release. The prompt now supports the following :

Operators: {}, {}-{}, {}:{}: Will download the songs from the list. {}.p, {}-{}.p, {}:{}.p: Will stream the songs from the list. {}.l: will show the lyrics

Commands: .conf : show the config (priority: command line > config file > default) .ls: show the current song list. .save {name}: save the current song list. .lsave: list all the saved song list. .load {name}: load the saved song list.

Command which may be added: .cls or .clear: clears the screen .del {name}: delete a saved song list. .set {option} {value}: on the fly config update. .gen: write the current config to config file. {}.i: show the info of the song. {}.u: show download url of the song. {}.t: show the thumbnail url of the song.