docs: add note about /var/lib/hwclock to TODO

Signed-off-by: Karel Zak <kzak@redhat.com>

Documentation/TODO

@@ -1,6 +1,13 @@
 
  Note that items with (!) have high priority.
 
+hwlock
+------
+ - use /var/lib/hwclock/drift to store hw-clock drift numbers.
+ - use /etc/adjtime as read-only for UTC/LOCAL information only
+ - the /var/lib/hwclock/drift should be implemented backwardly compatible,
+   it means use the file only if exists, otherwise follow /etc/adjtime 
+
 bash completion
 ---------------
  - file per command in shel-completion/bash/