Add axum-extra crate (#536)

* Add `axum-extra` crate

Empty for now but now we have a place to put stuff.

I'll make a PR for moving over https://github.com/davidpdrsn/axum-resource.

* remove `authors` field from `Cargo.toml`s

axum-extra/README.md

@@ -0,0 +1,45 @@
+# axum-extra
+
+[![Build status](https://github.com/tokio-rs/axum/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/tokio-rs/axum-extra/actions/workflows/CI.yml)
+[![Crates.io](https://img.shields.io/crates/v/axum-extra)](https://crates.io/crates/axum-extra)
+[![Documentation](https://docs.rs/axum-extra/badge.svg)](https://docs.rs/axum-extra)
+
+Extra utilities for [`axum`].
+
+More information about this crate can be found in the [crate documentation][docs].
+
+## Safety
+
+This crate uses `#![forbid(unsafe_code)]` to ensure everything is implemented in 100% safe Rust.
+
+## Minimum supported Rust version
+
+axum-extra's MSRV is 1.54.
+
+## Getting Help
+
+You're also welcome to ask in the [Discord channel][chat] or open an [issue]
+with your question.
+
+## Contributing
+
+:balloon: Thanks for your help improving the project! We are so happy to have
+you! We have a [contributing guide][contributing] to help you get involved in the
+`axum` project.
+
+## License
+
+This project is licensed under the [MIT license][license].
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in `axum` by you, shall be licensed as MIT, without any
+additional terms or conditions.
+
+[`axum`]: https://crates.io/crates/axum
+[chat]: https://discord.gg/tokio
+[contributing]: /CONTRIBUTING.md
+[docs]: https://docs.rs/axum-extra
+[license]: /axum-extra/LICENSE
+[issue]: https://github.com/tokio-rs/axum/issues/new