second commit :S

src/Components/Header/Header.js

@@ -1,12 +1,14 @@
 import React from 'react';
 
 import styles from './Header.scss';
+import classNames from 'classnames/bind';
+const cx = classNames.bind(styles);
 
-const Header = ({ activeEditors, toggleEditors }) => {
+const Header = ({ activeEditors, toggleEditors, showHistorySidebar, toggleHistorySidebar }) => {
   return (
     <div className={ styles.header }>
       <div className={ styles.headerItem }><i>Beautiful</i> Atlas</div>
-      <div className={ `${styles.headerItem} ${styles.editorToggle}` }>
+      <div className={ cx('headerItem', 'editorToggle') }>
         <span
           onClick={toggleEditors.bind(this, 'MARKDOWN')}
           className={ activeEditors.includes('MARKDOWN') ? styles.active : '' }
@@ -16,9 +18,19 @@ const Header = ({ activeEditors, toggleEditors }) => {
           className={ activeEditors.includes('TEXT') ? styles.active : '' }
         >Text</span>
       </div>
-      <div className={ styles.headerItem }>Versions</div>
+      <div className={ cx('headerItem', 'sidebar') }>
+        <span
+          onClick={toggleHistorySidebar}
+          className={ showHistorySidebar ? styles.active : '' }
+        >History</span>
+      </div>
     </div>
   );
 };
 
+Header.propTypes = {
+  showHistorySidebar: React.PropTypes.bool.isRequired,
+  toggleHistorySidebar: React.PropTypes.func.isRequired,
+}
+
 export default Header;

src/Components/Header/Header.scss

@@ -40,6 +40,13 @@
     }
   }
 
+  .sidebar {
+    span {
+      margin-right: 12px;
+      cursor: pointer;
+    }
+  }
+
   .active {
     text-decoration: underline;
     text-decoration-color: #fff;

src/Components/HistorySidebar/HistorySidebar.js

@@ -0,0 +1,17 @@
+import React from 'react';
+
+import styles from './HistorySidebar.scss';
+
+class HistorySidebar extends React.Component {
+  render() {
+    return (
+      <div className={ styles.container }>
+        <div className={ styles.header }>
+          Revisions
+        </div>
+      </div>
+    );
+  }
+};
+
+export default HistorySidebar;

src/Components/HistorySidebar/HistorySidebar.scss

@@ -0,0 +1,12 @@
+.container {
+  width: 200px;
+
+  background-color: #FBFBFB;
+  border-left: 1px solid #f0f0f0;
+
+  font-size: 12px;
+}
+
+.header {
+  padding: 20px 10px;
+}

src/Components/HistorySidebar/index.js

@@ -0,0 +1,2 @@
+import HistorySidebar from './HistorySidebar';
+export default HistorySidebar;