Update readme with information about tonemap functionality

README.md

@@ -2,7 +2,7 @@
 SupMover - Shift timings and Screen Area of PGS/Sup subtitle
 
 # Usage
-`SupMover (<input.sup> <output.sup>) [delay (ms)] [crop (<left> <top> <right> <bottom>)] [resync (<num>/<den> | multFactor)] [add_zero]`
+`SupMover (<input.sup> <output.sup>) [delay (ms)] [crop (<left> <top> <right> <bottom>)] [resync (<num>/<den> | multFactor)] [add_zero] [tonemap <perc>]`
 
 `SupMover (<input.sup> <output.sup> <ms>)` old syntax, kept for backward compatibility
 
@@ -19,6 +19,8 @@ SupMover - Shift timings and Screen Area of PGS/Sup subtitle
   * If both modes are selected the delay will be adjusted if it comes before the resync parameter, for example if the program is launched with `delay 1000 resync 1.001` it will be internally adjusted to 1001ms, instead if it's launched with `resync 1.001 delay 1000` it will not
 * add_zero
   * Some media players (especially Plex) don't correctly sync `*.sup` subtitles.  They seem to ignore any delay before the first 'display set'.  This option adds a dummy 'display set' at time 0 so subsequent timestamps are correctly interpreted.
+* tonemap
+  * Change the brightness of the subtitle applying the specified percentage factor to all the palette's luminance value, similar to https://github.com/quietvoid/subtitle_tonemap , the percentage must be specified as a decimal value with 1 as 100%, it can be bigger than 1 to increase brightness
 
 # Build instruction
 ```