README: document PREGENERATED_BBA_PATH
README: document PREGENERATED_BBA_PATH

Signed-off-by: Sean Cross <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@xobs.io>

SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr