Update README.md

Author: bkrem

README.md

@@ -2,6 +2,9 @@
 [![Build Status](https://travis-ci.org/bkrem/react-d3-tree.svg?branch=master)](https://travis-ci.org/bkrem/react-d3-tree)
 [![Coverage Status](https://coveralls.io/repos/github/bkrem/react-d3-tree/badge.svg?branch=master)](https://coveralls.io/github/bkrem/react-d3-tree?branch=master)
 
+> :construction: :construction:  
+> This library is work-in-progress, meaning breaking changes/major version bumps are likely until indicated otherwise.  
+
 React D3 Tree is a [React](http://facebook.github.io/react/) component that lets you represent hierarchical data (e.g. ancestor trees, organisational structure, package dependencies) as an animated & interactive tree graph by leveraging [D3](https://d3js.org/)'s `tree` layout.
 
 ## Demo