added usage

README.md

@@ -1,3 +1,19 @@
 # retro-stand
 
-Retroactively produce a stand output hdf5 from the written data files
+```
+usage: retro-stand.py [-h] [-o OUTPUT] [-c CHANNELS] base
+
+Retroactively produce a stand output hdf5 from the written data files
+
+positional arguments:
+  base                  base folder (e.g., /sf/instrument/data/p12345/raw/)
+
+optional arguments:
+  -h, --help            show this help message and exit
+  -o OUTPUT, --output OUTPUT
+                        output file name, if not specified no output is written (default: None)
+  -c CHANNELS, --channels CHANNELS
+                        channels file name, ascii file where each line is: column name = channel name (default:
+channels.txt)
+```
+