message: Add screenshot to `README` of `color_palette`
diff: -## Color Palette
  +## Color palette
  A color palette generator, based on a user-defined root color.
  +<div align="center">
  + <a href="https://gfycat.com/dirtylonebighornsheep">
  + <img src="https://github.com/hecrj/iced/raw/1a8d253611d3796b0a32b2f096bb54565a5292e0/examples/color_palette/screenshot.png">
  + </a>
  +</div>
  +
  You can run it with `cargo run`:
  ```
  