update docs to warn about `plugin` incompat with using in a Rust program that also links with libsqlite-sys
The reason to separate the plugin code from the rest of the crate is because if there's a sqlite entrypoint defined, as it is in the extension, there will be multiple entrypoints with that name defined in the process once you load the extension into sqlite itself, resulting in a confusing runtime panic. It's important to not enable that feature if you're using it as a cargo dependency.
Maybe also put a warning in the cargo file?