Readme.txt: Update description of ISM Metadata files

The description of the ISM Metadata files in readme.txt was not updated in the readme.txt:

These are comma separated files (csv) which indicate the per object position
in the format:
frame index, azimuth, elevation, distance (unit circle), spread, gain

with the following meaning:
| Parameter   | format, value range | meaning
----------------------------------------------------------------------
| frame index | int, [0, INT_MAX]   | associated IVAS frame
----------------------------------------------------------------------
| azimuth     | float, [-180,180[   | azimuth; positive indicates left
----------------------------------------------------------------------
| elevation   | float, [-90,90]     | elevation; positive indicates up
----------------------------------------------------------------------
| distance    | float, tbd          | default: 0
----------------------------------------------------------------------
| spread      | float, [0,360]      | spread in angles from 0 ... 360
----------------------------------------------------------------------
| gain        | float, [0,1]        | gain
----------------------------------------------------------------------

The row mentioning frame index should be removed to match the actual format.