Improve postcss-nesting docs around `@nest`

[Antonio-Laguna]

What would you want to propose?

I think that README on postcss-nesting should have some mention of @nest and how it should behave since it's not immediately clear and you'd have to research the spec yourself.

Suggested solution

I think a section around @nest what's wrong vs what's right (which could also benefit throughout other areas) so developers using the plugin don't have to read the spec and avoid pitfalls.

This is a common mistake:

.some-selector {
	@nest {
		.parent-selector & {
			margin-top: 0;
		}
	}
}

Additional context

No response

Validations

• [X] Follow our Code of Conduct
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Would you like to open a PR for this feature?

• [X] I'm willing to open a PR