My understanding based on a presentation from Dr. Nic is that Puma is a straight fork of Mongrel since Mongrel had threading from the beginning but it is essentially unsupported now.

This commit lends to that story as well: https://github.com/evanphx/puma/commit/1888887d8ff8fdf4954c3...

I agree though, would be nice to mention its heritage in the read me.