clean up README.md
clean up README.md

influxdb/levigo,jmhodges/levigo_leveldb_1.4,dckit/rocksdb,influxdata/rocksdb,alberts/gorocks,DanielMorsing/rocksdb,elijahandrews/levigo,influxdb/hyperleveldb-go,evanphx/levigo,influxdb/rocksdb,mikefaille/rocksdb-go,jmhodges/levigo,influxdata/hyperleveldb-go,chanxuehong/goleveldb