docs/reference/micropython2_migration.rst: Add info about os and vfs.

Signed-off-by: Damien George <damien@micropython.org>

docs/reference/micropython2_migration.rst

@@ -71,4 +71,11 @@ which is useful for development and testing.
 Changes
 ~~~~~~~
 
-*None yet*
+Introduction of a new module :mod:`vfs`.  The following functions and
+classes have moved out of :mod:`os` to :mod:`vfs`:
+- `os.mount`
+- `os.umount`
+- `os.VfsFat`
+- `os.VfsLfs1`
+- `os.VfsLfs2`
+- `os.VfsPosix`