todo: Document further virt-sparsify improvements.

TODO

@@ -611,3 +611,11 @@ opened with O_CLOEXEC.  Therefore we need to examine every call to:
  - socket, socketpair
  - accept (see also: accept4)
  - signalfd, timerfd, epoll_create
+
+virt-sparsify enhancements
+--------------------------
+
+TMPDIR should be checked to ensure that we won't run out of space
+during the conversion, since current behaviour is very bad when this
+happens (it usually causes virt-sparsify to hang).  This requires
+writing a small C binding to statvfs for OCaml.