[Rust]: Set "readme" metadata for each package separately

With 735c7e2668d0c17f18a3e4a7143a579674fd18a2 most of the package/crate metadata has been moved to the workspace Cargo.toml.

Since the "readme" property value is the same for all packages (readme = "README.md") it has been moved to the workspace too.

Today when publishing 0.16.0 I've noticed that all landing/index pages, e.g. https://crates.io/crates/apache-avro/0.16.0, actually show the README.md of the parent/workspace instead of the packages' README.md files.