feat(docs): README を追加

README.md

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+# nbem
+
+## Description
+
+nbem is for intuitively write the class name of BEM method.
+
+## Installation
+
+```
+npm i nbem
+```
+
+## Usage
+
+Use with node.js, browserify, webpack:
+
+```js
+var nbem = require(nbem);
+var bem = nbem();
+```
+
+'&' is indicates the nests.
+
+```js
+bem('header'); // => "header"
+bem('&nav'); // => "header__nav"
+bem('&&item'); // => "header__nav__item"
+```
+
+':' is indicates the modifier.
+
+```js
+bem('header:global'); // => "header--global"
+bem('&nav'); // => "header--global__nav"
+```
+
+Customize separator:
+
+```js
+var config = require(nbem).config;
+config.setSeparator('-', '_'); // element, modifier
+header('header:global'); // => "header-global"
+header('&title') // => "header-global_title"
+```
+
+## Example
+
+in React of ES6
+
+```js
+import nbem from 'nbem';
+
+export default class Header extends React.Component {
+  render() {
+    const bem = nbem();
+    return (
+      <header className={bem('header')}>
+        <h1 className={bem('&title')}>Title</h1>
+        <p className={bem('&description')>Description</p>
+      </header>
+    );
+  }
+}
+```
+
+Render:
+```html
+<header class="header">
+  <h1 class="header__title">Title</h1>
+  <p class="header__description">Description</p>
+</header>
+```