add a button with react code example

Example 1: react button onclick

<button onClick={sayHello}>Click</button>

Example 2: react event listener

onClick onContextMenu onDoubleClick onDrag onDragEnd onDragEnter onDragExit
onDragLeave onDragOver onDragStart onDrop onMouseDown onMouseEnter onMouseLeave
onMouseMove onMouseOut onMouseOver onMouseUp