Suggest users to use the `dev` profile while in development

For reasons discussed in [this guide of another random Rust project](https://bevy-cheatbook.github.io/pitfalls/performance.html), it could be considered bad practice to use `release` mode during development of Rust projects (especially ones with high complexity, e.g. GUIs). I'd suggest changing the `Cargo.toml` and `README.md` files to reflect this ideology.