reduce rainbow checkbox (#824)

README.md

@@ -187,6 +187,7 @@ OUTPUT SETTINGS:
                         Split output into multiple files. 0: Don't split 1: Automatic mode 2: Consider every subdirectory as separate volume [Default=0]
   --spreadshift         Shift first page to opposite side in landscape for two page spread alignment
   --norotate            Do not rotate double page spreads in spread splitter option.
+  --reducerainbow       Reduce rainbow effect on color eink by slightly blurring images
 
 CUSTOM PROFILE:
   --customwidth CUSTOMWIDTH