Add documentation for --patch-from

programs/README.md

@@ -172,6 +172,7 @@ Advanced compression arguments :
 --long[=#]: enable long distance matching with given window log (default: 27)
 --fast[=#]: switch to very fast compression levels (default: 1)
 --adapt : dynamically adapt compression level to I/O conditions
+--patch-from=FILE : specify the file to be used as a reference point for zstd's diff engine
  -T#    : spawns # compression threads (default: 1, 0==# cores)
  -B#    : select size of each job (default: 0==automatic)
 --single-thread : use a single thread for both I/O and compression (result slightly different than -T1)