Add changelog entry for byHash lookups.
Add changelog entry for byHash lookups.

Extend the Namecoin release notes for the upcoming 0.21 version to
mention the new name-hash index and byHash lookups.

domob1812/namecore,domob1812/namecore,domob1812/namecore,namecoin/namecoin-core,namecoin/namecore,namecoin/namecoin-core,pataquets/namecoin-core,pataquets/namecoin-core,pataquets/namecoin-core,pataquets/namecoin-core,namecoin/namecoin-core,domob1812/namecore,namecoin/namecore,namecoin/namecoin-core,pataquets/namecoin-core,namecoin/namecore,namecoin/namecore,namecoin/namecoin-core,namecoin/namecore,domob1812/namecore,namecoin/namecoin-core,pataquets/namecoin-core,namecoin/namecore,domob1812/namecore