add some description of Unsafe to the readme
add some description of Unsafe to the readme

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