docs(caifile.py): update module docstring

Signed-off-by: Bofu Chen (bafu) <bofu@numbersprotocol.io>

Author: bafu

codestream_parser/caifile.py

@@ -1,6 +1,13 @@
 #!/usr/bin/python
 
-# $Id: caifile.py,v 1.75 2020/10/15 12:14:27 thor Exp $
+'''Parse CAI injections to a JSON object.
+
+caifile.py was modified based on jp2file.py to leverage the existing parsing
+functions.
+
+The global Cai_boxes list receives the parsing results in multiple parsing
+funcitons.
+'''
 
 import getopt
 import json