I've added an ?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.