Docs about `yii\bootstrap\Html` added

Html.php

@@ -8,7 +8,7 @@
 namespace yii\bootstrap;
 
 /**
- * Html is an enhanced version of [[\yii\helpers\Html]] helper class dedicated to Bootstrap needs.
+ * Html is an enhanced version of [[\yii\helpers\Html]] helper class dedicated to the Bootstrap needs.
  * This class inherits all functionality available at [[\yii\helpers\Html]] and can be used as substitute.
  *
  * Attention: do not confuse [[\yii\bootstrap\Html]] and [[\yii\helpers\Html]], be careful in which class

docs/guide/README.md

@@ -20,6 +20,7 @@ Usage
 ----- 
 
 * [Yii widgets](usage-widgets.md)
+* [Html helper](helper-html.md)
 
 Additional topics
 -----------------

docs/guide/helper-html.md

@@ -0,0 +1,30 @@
+Html helper
+===========
+
+Bootstrap introduces many consistent HTML constructions and skeletons, which allow creating different visual effects.
+Only the most complex of them are covered by widgets provided with this extension. The rest should be composed manually
+using direct HTML composition.
+However, several special Bootstrap markup cases are covered by [[\yii\bootstrap\Html]] helper.
+[[\yii\bootstrap\Html]] is an enhanced version of the regular [[\yii\helpers\Html]] dedicated to the Bootstrap needs.
+It provides several useful methods:
+
+ - `icon()` - allows rendering of Glyphicon icons
+ - `staticControl()` - allows rendering of form "static controls"
+
+[[\yii\bootstrap\Html]] inherits all functionality available at [[\yii\helpers\Html]] and can be used as substitute,
+so you don't need them both inside your view files.
+For example:
+
+```php
+<?php
+use yii\bootstrap\Html;
+?>
+<?= Button::widget([
+    'label' => Html::icon('approve') . Html::encode('Save & apply'),
+    'encodeLabel' => false,
+    'options' => ['class' => 'btn-primary'],
+]); ?>
+```
+
+> Attention: do not confuse [[\yii\bootstrap\Html]] and [[\yii\helpers\Html]], be careful in which class
+  you are using inside your views.