Improve readme

README.md

-# btrfs-snapshots-diff
+btrfs-snapshots-diff
+====================
+
+About
+-----
+btrfs-snapshots-diff.py is a  simple Python script that displays the differences 
+between 2 Btrfs snapshots (from the same subvolume obviously).
 
-Displays differences in 2 Btrfs snapshots (from the same subvolume
-obviously).
 Uses btrfs send to compute the differences, decodes the stream and 
 displays the differences.
+
 Can read data from parent and current snapshots, or from diff
 file created with:
 btrfs send -p parent chid --no-data -f /tmp/snaps-diff
 
+Usage
+-----
+
+    usage: btrfs-snapshots-diff.py [-h] [-p PARENT] [-c CHILD] [-f FILE] [-v]
+    
+    Display differences between 2 Btrfs snapshots
+    
+    optional arguments:
+      -h, --help            show this help message and exit
+      -p PARENT, --parent PARENT
+                            parent snapshot (must exists and be readonly)
+      -c CHILD, --child CHILD
+                            child snapshot (will be created if it does not exist)
+      -f FILE, --file FILE  diff file
+      -v, --verbose         increase verbosity
+
+Requirements
+------------
+No requirements besides Python-2!
+
+
+Made in beautiful Tahiti (French Polynesia) by [SysNux](http://wwww.sysnux.pf/).
+
 Copyright (c) 2016 Jean-Denis Girard <jd.girard@sysnux.pf>