You should read and consider the design notes of course.
Generally I like to use forks for feature additions, and only use hg branches for things like a stable branch. So if you want to add a feature, you should fork/clone the repository and make those changes. I appreciate some note (in the description field) about what your fork’s purpose is. It might simply be “to mess around with Silver Lining”, but I still appreciate the note.
The main repository is for integration. I’m not shy about giving out write access to this repository, if you are making consistent contributions it’s easier for both of us if you can simply push directly to the main branch. This doesn’t remove the utility of forks; if you are doing something that will take a while to complete or you aren’t sure if it’s in line with Silver Lining’s design, it’s still reasonable to do your own fork.