added brief description of forms

docs/guide/form.md

 Working with forms
 ==================
 
+The primary way of using forms in Yii is [[\yii\widgets\ActiveForm]]. It should be preferred when you have a model
+behind a form. Additionally there are some useful methods in [[\yii\helpers\Html]] that are typically used for adding
+buttons and help text.
+
+First step creating a form is to create a model. It can be either Active Record or regular Model. Let's use regular
+login model as an example:
+
+```php
+use yii\base\Model;
+
+class LoginForm extends Model
+{
+	public $username;
+	public $password;
+
+	/**
+	 * @return array the validation rules.
+	 */
+	public function rules()
+	{
+		return [
+			// username and password are both required
+			['username, password', 'required'],
+			// password is validated by validatePassword()
+			['password', 'validatePassword'],
+		];
+	}
+
+	/**
+	 * Validates the password.
+	 * This method serves as the inline validation for password.
+	 */
+	public function validatePassword()
+	{
+		$user = User::findByUsername($this->username);
+		if (!$user || !$user->validatePassword($this->password)) {
+			$this->addError('password', 'Incorrect username or password.');
+		}
+	}
+
+	/**
+	 * Logs in a user using the provided username and password.
+	 * @return boolean whether the user is logged in successfully
+	 */
+	public function login()
+	{
+		if ($this->validate()) {
+			$user = User::findByUsername($this->username);
+			return true;
+		} else {
+			return false;
+		}
+	}
+}
+```
+
+In controller we're passing model to view where Active Form is used:
+
+```php
+use yii\helpers\Html;
+use yii\widgets\ActiveForm;
+
+<?php $form = ActiveForm::begin([
+	'id' => 'login-form',
+	'options' => ['class' => 'form-horizontal'],
+]) ?>
+	<?= $form->field($model, 'username') ?>
+	<?= $form->field($model, 'password')->passwordInput() ?>
+
+	<div class="form-group">
+		<div class="col-lg-offset-1 col-lg-11">
+			<?= Html::submitButton('Login', ['class' => 'btn btn-primary']) ?>
+		</div>
+	</div>
+<?php ActiveForm::end() ?>
+```
+
+In the code above `ActiveForm::begin()` not only creates form instance but marks the beginning of the form. All the content
+that is located between `ActiveForm::begin()` and `ActiveForm::end()` will be wrapped with appropriate `<form>` tag.
+Same as with any other widget you can specify some options passing an array to `begin` method. In our case we're adding
+extra CSS class and specifying ID that will be used in the tag.
+
+In order to insert a form field along with its label all necessary validation JavaScript we're calling `field` method
+and it gives back `\yii\widgets\ActiveField`. It it's echoed directly it creates a regular input. In case you want to
+customize it you can add a chain of additional methods:
+
+```php
+<?= $form->field($model, 'password')->passwordInput() ?>
+
+// or
+
+<?= $form->field($model, 'username')->textInput()->hint('Please enter your name')->label('Name') ?>
+```