I've added an
[[external_special_remote_protocol/export_and_import_appendix]]
to the [[external_special_remote_protocol]] which documents how
the protocol might be extended to allow for importing from external special
remotes.

Feel this needs more thought. It's complicated by there already
being an interface that only supports export, and import needing all
the same operations, but with more checks that the content has not been
modified behind git-annex's back. Unifying them at the protocol level would
be possible, but perhaps more confusing.