Wishlist: delimit docs-only section

[chrysn]

When a crate's documentation contains a point from where on it is not practical to include in the README any more, it would be convenient to add a magic marker (say, <!-- END OF README PART -->) into the text and cut it there.

For example, in https://gitlab.com/etonomy/riot-sys, the main part is about building and integration which is all good, but where it cuts to what is in the main crate vs. what is in pub modules, this belongs into the docs but not the README.