Change command naming: `./siac renter export contracts` to `./siac renter export rankings-data` and expand the help messages.
Change command naming: `./siac renter export contracts` to `./siac renter export rankings-data` and expand the help messages.

mtlynch/Sia,NebulousLabs/Sia,MrStrong/Sia,ChrisSchinnerl/Sia,mharkus/Sia,mtlynch/Sia,ChrisSchinnerl/Sia,mharkus/Sia,ericflo/Sia,MrStrong/Sia,NebulousLabs/Sia,ericflo/Sia