The `KeepDirectoryArchiveImportHistory` option allows us to keep a history
of imported directory archive files to know which files have been imported
before. This history can be useful when importing from a changing source
to avoid importing descriptors over and over again, but it can be
confusing to users who don't know about it.
However, this approach does not work for importing cached descriptors from
a local Tor directory, because descriptors are contained in files like
cached-descriptors. Filenames never change here, just contents.
The new approach is to also keep the file modification time in our history
and check whether files have been modified after we last read them. This
lets us import descriptors from a local Tor directory, too.
Also update the README.
