Add WIP warning

Author: fritz-c

README.md

@@ -1,5 +1,7 @@
 # React Sortable Tree
 
+__Note: This is a work in progress; most of the features are not yet implemented.__
+
 [DEMO](https://fritz-c.github.io/react-sortable-tree/)
 
 ### Features

src/examples/basicExample/app.js

@@ -86,6 +86,7 @@ const App = React.createClass({
                 </section>
 
                 <section className={styles['main-content']}>
+                    <span style={{ color: 'firebrick' }}>Note: This is a work in progress; most of the features are not yet implemented.</span>
                     <h3>Demo</h3>
 
                     <SortableTree