Add a simple example to the README

[philpax]

Hiya!

Thanks for the crate; I've successfully spruced up my egui UI with it 🙂

That being said, it took a bit of looking around to figure out how to use it; I had to look through egui-dashboard-template to find the relevant code, and found that a bit difficult as it doesn't document where things are.

To help with that, I'd suggest:

• adding a simple example of how to use the crate in the README - this can just be a few lines where the trait's imported and set_style is called with Aesthetix::custom_style on one of the styles
• indicating that egui-dashboard-template is a more complete example with runtime theme selection
• documenting egui-dashboard-template further so that it's obvious where a developer might look to find egui-aesthetix-related code