update README docs

README.md

@@ -36,19 +36,28 @@ use responsibly.
 ## Usage
 
 ```
-usage: impatient [-h] [-f FINAL] [-i INTERVAL] [-w WINDOW] (-p PATH | -c COMMAND)
+usage: impatient [-h] [-f FINAL] [-i INTERVAL] [-w WINDOW] [-d DECAY]
+                 (-p PATH | -c COMMAND)
 
 Display progress and time estimates for arbitrary tasks
 
 optional arguments:
   -h, --help            show this help message and exit
   -f FINAL, --final FINAL
-                        Expected final size/value at completion, optionally with a K/M/G/T suffix
+                        Expected final size/value at completion, optionally
+                        with a K/M/G/T suffix
   -i INTERVAL, --interval INTERVAL
                         Interval in seconds between samples (default 10)
   -w WINDOW, --window WINDOW
-                        Number of samples to keep for the sliding window (default 100)
+                        Number of samples to keep for the sliding window
+                        (default 100)
+  -d DECAY, --decay DECAY
+                        Decay coefficient for older samples (default 1). Must
+                        be between 0 and 1 inclusive. The lower this is, the
+                        more responsive/swingy the estimate will be.
   -p PATH, --path PATH  Track total disk usage of a given path
   -c COMMAND, --command COMMAND
-                        Track value returned by a shell command; this should return a single number, optionally with a K/M/G/T suffix
+                        Track value returned by a shell command; this should
+                        return a single number, optionally with a K/M/G/T
+                        suffix
 ```