Improve README.md

Signed-off-by: David Scott <[email protected]>

README.md

@@ -1,4 +1,18 @@
 ocaml-mbr
 =========
 
-A simple library for manipulating Master Boot Records
+A simple library for manipulating Master Boot Records. The
+primary purposes of this library are:
+  1. to create bootable disk images creating
+     [mirage](http://www.openmirage.org/) kernels
+  2. for mirage kernels to read the partition tables on
+     attached disks
+
+To do items
+-----------
+
+* It's easy to specify nonsense, for example overlapping
+  partition entries. We could do with a way to represent a
+  disk as a set of non-overlapping extents which could be
+  nested (partition / LVM volume / filesystem / file)
+