docs: update README.md (#100)

Add information on the stylesheet.

Author: warent

README.md

@@ -44,6 +44,11 @@ React Tooltip
 var Tooltip = require('rc-tooltip');
 var React = require('react');
 var ReactDOM = require('react-dom');
+
+// By default, the tooltip has no style.
+// Consider importing the stylesheet it comes with:
+// 'rc-tooltip/assets/bootstrap_white.css'
+
 ReactDOM.render(<Tooltip placement="left" trigger={['click']} overlay={<span>tooltip</span>}><a href='#'>hover</a></Tooltip>, container);
 ```