add a link to README answering common question
add a link to README answering common question

Signed-off-by: Dustin DeWeese <ecf426c960ed624a3b3143c1895d49c821531d9b@gmail.com>

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