[…] people tend not to read these things […]
So, I guess the question becomes "what is a good place for people to find this info "?
That question is insightful.
People tend not to read these things because they are not cloning when they are at that page. The information should be at where they are when they are cloning: the swipl-devel GitHub repository. The information, the person, and the task must be near to each other in space and time. Ideally, the information is presented right where people need it when they need it.
The question becomes “Where are they when they need that information?”
The answer: They probably are at swipl-devel at GitHub, after searching for “swi prolog source code” in Google. (I may be wrong. You may have a more accurate answer from the website statistics.)
Thus, I think the best place for that information is the README.md file in swipl-devel, because people will be looking at that when they are cloning. The readme is as close as possible to the “Fork” and “Clone” button as GitHub allows. The readme is the only place that is zero clicks away from where people are when they are cloning.
Also, we can assume that people want to build the source right after they clone it, so the information about building should be placed right after the information about cloning. Then, they will want to install it, run it, learn about it, play with it, write big programs in it, contribute to it, and so on. Thus the sequence of information in README.md should follow that most likely sequence of tasks done by a new contributor.