Add documentation for the available features

[rodya-mirov]

Description

There are a lot of features available in specs, and none of these features are documented in any useful. The Cargo.toml is not self-documenting (I think this is more a failing of Cargo than this project) and they are not specifically mentioned in the documentation. I just spent a very frustrating 90 minutes trying to figure out why specs::saveload is not a real module (in the end, to compile the sample code, I needed to add "serde" and "uuid_entity").

Motivation

I just spent a very frustrating 90 minutes trying to figure out why specs::saveload is not a real module.

Drawbacks

Extra documentation has to be maintained. But this seems manageable.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Feel free to comment if this is still relevant.