Update readme.txt

-rvf File           : Reference vector specified by external trajectory file

                      works only in combination with '-otr ref_vec' and 'ref_vec_lev' modes

-render_config File : Renderer configuration option File

-non_diegetic_pan   : panning mono non-diegetic sound to stereo -90<= pan <=90,

-non_diegetic_pan P : panning mono non-diegetic sound to stereo -90<= P <=90,

                      left or l or 90->left, right or r or -90->right, center or c or  0->middle

-q                  : Quiet mode, no frame counter

                      default is deactivated

|                                    | if Flag is set to 1, panning gain is specified by azimuth.

|                                    | Value between [-90,90], 90 left, -90 right, 0 center

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*Read as float value for convenience, but used as flag internally.

*Read as float value for convenience, but used as an integer flag internally.

The metadata reader accepts 1-8 values specified per line. If a value is not specified, the default

value is assumed.