Comparison with cpm-cmake/CPM.cmake

[LecrisUT]

At first glance it is rather difficult to judge the differences between this project and CPM, which is currently more popular. So whenever possible, can a comparison with that be documented? Mostly I think it would be helpful to point out:

• Differences in targets and philosophy. E.g. CPM is strictly aimed at downloading cmake based projects, and it does not aim to create any central repository of this. As I understand pmm aims at bridging with other projects as well and also make a more centralized repository of cmake projects.
• Differences in syntax and options. E.g. in CPM you can directly pass the cmake options to the dependent when you add it to the project, you can prioritize system installed packages, etc. How do you do similar actions in ppm?
• Differences in implementations. CPM is a single-file cmake dependency, that you can add it either manually or using an automatic function.
• Differences in features.