nextpnr doc for router and placer options

[WillGreen]

nextpnr is a wonderful piece of software, but there isn't much information on the different router and placer options.

I have a design which doesn't meet timing and where the routing time for the critical path varies enourmously depending on the seed (by more than a factor of 2). I've looked at nextpnr-ecp5 --help and there are many options I could change but there's little information to go on.

Could you add a doc explaining the placer and router options and offering some advice on how to optimise timing with them?

In the medium term I'd love to see nextpnr documented in the same style as Yosys, but I understand this is a significant undertaking.

TIA