Update README

Align with CustomStateSet API

Author: nkabrown

README.md

@@ -152,7 +152,7 @@ class CheckedControl extends HTMLElement {
 
 ### State API
 
-`ElementInternals` exposes an API for creating custom states on an element. For instance if a developer wanted to signify to users that an element was in state `foo`, they could call `internals.states.set('--foo')`. This would make the element match the selector `:--foo`. Unfortunately in non-supporting browsers this is an invalid selector and will throw an error in JS and would cause the parsing of a CSS rule to fail. As a result, this polyfill will add states using the `state--foo` attribute to the host element, as well as a `state--foo` shadow part in supporting browsers.
+`ElementInternals` exposes an API for creating custom states on an element. For instance if a developer wanted to signify to users that an element was in state `foo`, they could call `internals.states.add('--foo')`. This would make the element match the selector `:--foo`. Unfortunately in non-supporting browsers this is an invalid selector and will throw an error in JS and would cause the parsing of a CSS rule to fail. As a result, this polyfill will add states using the `state--foo` attribute to the host element, as well as a `state--foo` shadow part in supporting browsers.
 
 In order to properly select these elements in CSS, you will need to duplicate your rule as follows: