Add Lit example to the Readme

[Luis Herranz]

Lit now supports Preact Signals, so it'd be great to share an example in the readme about how to use it with deepsignal.

References:

• https://lit.dev/blog/2023-10-10-lit-3.0/#preact-signals-integration
• https://stackblitz.com/edit/lit-and-deepsignal?file=src%2Fmy-element.js