update readme

README.md

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 # element-inspector.js
+
+[![Build Status](https://travis-ci.org/cou929/element-inspector.js.svg?branch=master)](https://travis-ci.org/cou929/element-inspector.js)
+
 Inspect DOM elements like developer tools of browser
+
+![demo](/example/demo.gif)
+
+## Example
+
+```js
+new ElementInspector({
+  // target element to enable inspector
+  targetSelector: 'body'
+});
+```
+
+## Constructor
+
+### `targetSelector`
+
+The target element to enable inspector. Required.
+
+### `onMousemove`
+
+Callback function of mousemove event. The argument of callback is event object.
+
+### `onOverlayClicked`
+
+Callback function of click event of overlay element, which indicates the dom of current mouse position. The argument of callback is event object.
+
+### `overlayBackgroundColor`
+
+Background color of overlay. Default is `rgba(102, 204, 255, 0.5)` (semitransparent light blue).
+
+## Instance methods
+
+### `showOverlay()`
+
+Show overlay.
+
+### `hideOverlay()`
+
+Hide overlay.
+
+## Test
+
+```
+npm test
+```