(tracking issue for https://github.com/mongodb/bson-rust/issues/141)

Currently, we don't have any system in place to document the non-default feature of the BSON library (e.g. "u2i", which allows serializing unsigned integers). We should update our documentation to show all features of the library and label the non-default features with their corresponding flags; https://docs.rs/tokio is a good example of how we should do this.