Hacker News Reader

you may want to take a look at http://www.kernel.org/pub/software/scm/git/docs/git-submodul.... mercurial lacks this as of now.

Mercurial has extension support, and one of them (used for example on the OpenJDK project - a "real" project of some size) is "Forest", which does just that: http://www.bitbucket.org/pmezard/hgforest-crew/overview/

there is no documentation at all...

Most things can be found on the Mercurial repository:

Not sure why there are no docs in the hgforest repository though - that seems like a pretty big oversight.