add keys to example template

    ### the individual items (concatenate_input: false) after previous pre-processing step

    ### Horizontally concatenate input items into one long file; default = false

    # concatenate_input: true

    ### if concatenation is applied, the following two keys can be used to add zeros before or after the items

    ### duration is specified in miliseconds

    # silence_pre: 2000

    # silence_post: 2000

    ### Specify the concatenation order in a list of strings. If not specified, the concatenation order would be

    ### as per the filesystem on the users' device

    ### Should only be used if concatenate_input = true