document that rebuilderd should be run on 64-bit archs

As you know in the beginning I tried to run rebuilderd for Debian i386 on Debian i386 but after a few days the db "exploded" and we lost many entries, which happened several times until I stopped running rebuilderd on i386 and instead ran rebuilderd for i386 on amd64.

Please document this limitation and recommend to run rebuilderd on 64bit architectures. Maybe even enforce this.