What you wrote is not true; the development of nMigen did not start at M-Labs, but rather it began its life as my personal project that was not coordinated with M-Labs in any way. I would appreciate it if you did not make assertions about events you have no knowledge of.

That being said, I don't see the utility of mentioning an outdated repository in the README of an up-to-date one. What you want to avoid is people thinking that m-labs/nmigen is the up-to-date source tree, right? So it would be nice to add a mention there.