Add libconfig reference to doc.
Add libconfig reference to doc.

tuomassiren/libbitcoin-server,libmetrocoin/libmetrocoin-server,tuomassiren/libbitcoin-server,RojavaCrypto/libbitcoin-server,libmetrocoin/libmetrocoin-server,tuomassiren/libbitcoin-server,libmetrocoin/libmetrocoin-server,RojavaCrypto/libbitcoin-server,RojavaCrypto/libbitcoin-server