Update README.md example to use leveldb.
Update README.md example to use leveldb.

SQLite is now fully deprecated and has been removed.  Spotted and reported
by @jolan.

Closes #3.

davecgh/btcd,jrick/dcrd,mathiasrw/btcd,pkothbauer/btcd,BitGo/prova,jrick/dcrd,jongillham/btcd,soapboxsys/ombfullnode,jrick/btcd,Cleawing/btcd,adiabat/btcd,ppcsuite/ppcd-old,Roasbeef/btcd,decred/dcrd,hectorj/btcd,DanielKrawisz/btcd,FactomProject/btcd,ppcsuite/ppcd-old,soapboxsys/ombfullnode,FactomProject/btcd,tb00/btcd,runeaune/btcd,cqswxj2013/btcd,toddfries/btcd,aakselrod/btcd,runeaune/btcd,hectorj/btcd,davecgh/btcd,jcmincke/dcrd,danoctavian/btcd,adiabat/btcd,jimmysong/btcd,stevenroose/btcd,jimmysong/btcd,CryptocurrencyCabal/htcd,Cleawing/btcd,ppcsuite/ppcd,jcmincke/dcrd,BitGo/prova,cqswxj2013/btcd,glv2/ppcd,Dirbaio/btcd,dajohi/btcd,jongillham/btcd,dan-da/btcd,btcsuitereleases/btcd,jrick/btcd,1d4Nf6/btcd,dajohi/btcd,wallclockbuilder/btcd,btcsuite/btcd,decred/dcrd,jrick/btcd,cjepson/btcd,Dirbaio/btcd,sysbot/btcd,wallclockbuilder/btcd,DanielKrawisz/btcd,jcmincke/dcrd,btcsuite/btcd,ppcsuite/ppcd,CryptocurrencyCabal/htcd,tb00/btcd,Dirbaio/btcd,stevenroose/btcd,aakselrod/btcd,dan-da/btcd,Roasbeef/btcd,Stash-Crypto/btcd,cjepson/btcd,sysbot/btcd,1d4Nf6/btcd,ltcsuite/ltcd,btcsuitereleases/btcd,dajohi/btcd,qchain/btcd,mathiasrw/btcd,qchain/btcd,Stash-Crypto/btcd,pkothbauer/btcd,danoctavian/btcd,toddfries/btcd,glv2/ppcd,ltcsuite/ltcd