message: Add note about resolver requirement
diff: @@ -69,6 +69,9 @@ Add `iced` as a dependency in your `Cargo.toml`:
  iced = "0.4"
  ```
  +If your project is using a Rust edition older than 2021, then you will need to
  +set `resolver = "2"` in the `[package]` section as well.
  +
  __Iced moves fast and the `master` branch can contain breaking changes!__ If
  you want to learn about a specific release, check out [the release list].
  