Adds README for code generation · amazon-ion/ion-cli@b84e8a2

The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2, actions-rs/toolchain@v1, actions-rs/cargo@v1. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v2, actions-rs/toolchain@v1, actions-rs/cargo@v1. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

unused import: `ErrorKind`

unused import: `crate::hex_reader::DigitState::ZeroX`

unused import: `Cursor`

unused import: `Read`

field `schema_system` is never read

method `get_schema_system` is never used

`ion-cli` (bin "ion") generated 6 warnings (run `cargo fix --bin "ion"` to apply 3 suggestions)

`ion-cli` (bin "ion" test) generated 5 warnings (5 duplicates)

unused import: `IonResult`

unused import: `enum_type::EnumType`

The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/