Add note about custom editing areas w/React 16

See zenoamaro#309

This _may_ be related to a change in React 16: https://github.com/facebook/react/blob/master/CHANGELOG.md#breaking-changes

README.md

@@ -372,6 +372,8 @@ class MyComponent extends React.Component {
 
 If you instantiate ReactQuill without children, it will create a `<div>` for you, to be used as the editing area for Quill. If you prefer, you can specify your own element for ReactQuill to use. Note that `<textarea>`s are not supported by Quill at this time.
 
+Note: Custom editing areas lose focus when using React 16 as a peer dep at this time ([bug](https://github.com/zenoamaro/react-quill/issues/309)).
+
 <details>
 
 ```jsx