Include @generated in generated schema to help rustfmt skip it

[wezm]

A feature was recently merged into rustfmt that makes it aware of @generated. This is an informal convention that tools can use to recognise generated files. This issue in the rustfmt repo discusses other tools that include or recoginse @generated. Notably Cargo includes it in a comment at the top of Cargo.lock:

# This file is automatically @generated by Cargo.
# It is not intended for manual editing.

It would be convenient if the schema.rs generated by diesel_cli included this text in a comment at the top of the file so that rustfmt will skip it, avoiding churn/manually reverting the changes made by cargo fmt.

Checklist

• I have already looked over the issue tracker for similar issues.
• This issue can be reproduced on Rust's stable channel.