Update documentation

Author: prichodko

README.md

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+# `@rehooks/window-scroll-position`
+
+> React hook for [Window](https://developer.mozilla.org/en-US/docs/Web/API/Window) scroll position
+
+> **Note:** This is using the new [React Hooks API Proposal](https://reactjs.org/docs/hooks-intro.html)
+> which is subject to change until React 16.7 final.
+>
+> You'll need to install `react`, `react-dom`, etc at `^16.7.0-alpha.0`
+
+## Install
+
+```sh
+yarn add @rehooks/window-scroll-position
+```
+
+## Usage
+
+```js
+import useWindowScrollPosition from '@rehooks/window-scroll-position'
+
+function MyComponent() {
+  // optionally you can pass options, those are default:
+  let options = {
+    throttle: 100,
+    passive: true,
+  }
+  let position = useWindowScrollPosition(options)
+  // position == { x: 0, y: 0 }
+  return <div />
+}
+```

example.js

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+import React from 'react'
+import { render } from 'react-dom'
+import useWindowScrollPosition from './'
+
+function App() {
+  let position = useWindowScrollPosition()
+  return (
+    <div
+      style={{
+        position: position.x === 0 ? 'fixed' : 'absolute', // is on top
+      }}
+    />
+  )
+}
+
+render(<App />, window.root)