Info about conversion to skiplist format for annotations.db and mailboxes.db

When upgrading from Kolab 2.3 the files annotations.db and mailboxes.db might be in Berkeley DB format.  Kolab 3 uses
skiplist, so we need to convert the files. Do that on the source
system, where we can work with both formats.
Merge pull request #12 from jh23453/patch-1

conversion to skiplist format for annotations.db and mailboxes.db