add database/sql to usage example
add database/sql to usage example

outrightmental/go-sqlx,johnsto/sqlx,snikch/sqlx,larsth/sqlx,StabbyCutyou/sqlx,proteneer/sqlx,jmoiron/sqlx,aiyi/sqlx,adcaes/sqlx,beni55/sqlx,cortex/sqlx,wmydz1/sqlx