message: Update `README` disclaimer
diff: A renderer-agnostic GUI library for Rust focused on simplicity and type-safety.
  Inspired by [Elm].
  -__Iced is in a very early stage of development.__ Many [basic features are still
  -missing] and there are probably _many_ bugs. [Feel free to contribute!]
  +__Iced is in a experimental stage.__ [Features are still missing],
  +[some optimizations are yet to be implemented], and there are probably _many_
  +bugs. [Feel free to contribute!]
  -[basic features are still missing]: https://github.com/hecrj/iced/issues?q=is%3Aissue+is%3Aopen+label%3Afeature
  +[Features are still missing]: https://github.com/hecrj/iced/issues?q=is%3Aissue+is%3Aopen+label%3Afeature
  +[some optimizations are yet to be implemented]: https://github.com/hecrj/iced/issues?q=is%3Aissue+is%3Aopen+label%3Aoptimization
  [Feel free to contribute!]: #contributing--feedback
  [![UI Tour - Coffee][gui_gif]][gui_gfycat]
  