Add note about CSS encapsulation

Author: koddsson

readme.md

@@ -49,6 +49,9 @@ document.addEventListener('image-crop-change', function (event){
 })
 ```
 
+## CSS encapsulation
+The elements HTML structure is initialized in a [Shadow DOM](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_shadow_DOM), so it is impossible to apply CSS to it. If you need to change the element's default style for any reason, you should open up a new issue (or a pull request!), describing your use case, and we'll work with you on solving the problem.
+
 ## Browser support
 
 Browsers without native [custom element support][support] require a [polyfill][]. Legacy browsers require various other polyfills. See [`examples/index.html`][example] for details.