Add Button command, and usage example in readme file

Add it under the contributing guide.

https://www.8bitcn.com/docs/components/button

It should just include the command:

`pnpm dlx shadcn@latest add @8bitcn/button`

and usage:

```
import { Button } from "@/components/ui/8bit/button"

<Button>Button</Button>
```

Also put a screenshot of the button