.db: add basic doc
.db: add basic doc

Signed-off-by: John McMaster <527ca5438b47432dedda4309be2f991b71bfb11c@gmail.com>

SymbiFlow/prjxray,SymbiFlow/prjxray,SymbiFlow/prjxray,SymbiFlow/prjxray,SymbiFlow/prjxray