update readme.txt wrt "-fr" option

                      EVS RTP Payload Format. The SDP parameter hf_only is required.

                      Reading RFC4867 AMR/AMR-WB RTP payload format is not supported.

-Tracefile TF       : VoIP mode: Generate trace file named TF

-fr5                : option to perform rendering + head-tracking with 5ms frame size

-fec_cfg_file       : Optimal channel aware configuration computed by the JBM

                      as described in Section 6.3.1 of TS26.448. The output is

                      written into a .txt file. Each line contains the FER indicator

                      Format files, the magic word in the mime file is used to determine

                      which of the two supported formats is in use.

                      default bitstream file format is G.192

-fr L                : render frame size in ms L=(5,10,20), default is 20					  

-hrtf File          : HRTF filter File used in BINAURAL rendering

-T File             : Head rotation specified by external trajectory File

-otr tracking_type  : Head orientation tracking type: 'none', 'ref', 'avg', 'ref_vec'

-of Format          : Audio Format of output file

                      Alternatively, it can be a custom loudspeaker layout File

-fs                 : Input sampling rate in kHz (16, 32, 48) - required only with raw PCM inputs

-fr L                : render frame size in ms L=(5,10,20), default is 20

-hrtf File          : Custom HRTF File for binaural rendering (only for binaural outputs)

-T File             : Head rotation trajectory File for simulation of head tracking (only for binaural outputs)

-otr tracking_type  : Head orientation tracking type: 'none', 'ref', 'avg' or `ref_vec` or `ref_vec_lev` (only for binaural outputs)