Update README.md

Author: phr00t

README.md

@@ -14,7 +14,7 @@ It works on a common line with arguments, which are all optional. If you just ru
 
 The arguments are:
 
-    input=[file/dir] output=[songs dir] duration=[seconds to process] tap=[true/false] tapsync=[offset time in seconds for tap, default: -0.11] hard=[true/false]
+    input=[file/dir] output=[songs dir] duration=[seconds to process] tap=[true/false] tapsync=[offset time in seconds for tap, default: -0.11] hard=[true/false] updatesm=[true/false]
 
 Example:
 
@@ -24,6 +24,8 @@ If you set tap=true, AutoStepper won't try and automatically calculate the BPM o
 
 It is best to let AutoStepper run through a whole bunch of music, and ones that it might not get exactly right -- to then pull out & use tap=true on them.
 
+updatesm=true will look for existing .sm stepfiles using the same filenames generated by AutoStepper. If found, it will take the offset & BPM from those files & just update the steps. This is useful for updating steps generated with old versions of AutoStepper, or changing the "hard" argument without having to recaculate BPM/offset times.
+
 You can also use the output as a base to further edit & perfect songs, with AutoStepper doing most of the dirty work.
 
 I will add it is optimized for pad use, not keyboard use (e.g. difficulty isn't high enough).