add arbitrary variants to docs

Author: dcastil

README.md

@@ -114,6 +114,16 @@ twMerge('[padding:1rem] p-8') // → '[padding:1rem] p-8'
 
 Watch out for mixing arbitrary properties which could be expressed as Tailwind classes. tailwind-merge does not resolve conflicts between arbitrary properties and their matching Tailwind classes to keep the bundle size small.
 
+### Supports arbitrary variants
+
+```ts
+twMerge('[&:nth-child(3)]:py-0 [&:nth-child(3)]:py-4') // → '[&:nth-child(3)]:py-4'
+twMerge('dark:hover:[&:nth-child(3)]:py-0 hover:dark:[&:nth-child(3)]:py-4')
+// → 'hover:dark:[&:nth-child(3)]:py-4'
+```
+
+Similarly to arbitrary properties, tailwind-merge does not resolve conflicts between arbitrary variants and their matching predefined modifiers, like `[&:focus]:ring` and `focus:ring`.
+
 ### Supports important modifier
 
 ```ts