Add new guide upload-file

1.  UploadedFile::getInstances($model, 'file'); sees only one file if not set ```'file[]'```

```php
<?= $form->field($model, 'file[]')->fileInput(['multiple' => '']) ?>
```

2. ```$form->errorSummary(..)``` not return all errors of file attribute, but only the last.
So I had to invent.

```php
if ($model->hasErrors()) {
    echo '<pre>';
    print_r($model->getErrors());
    echo '</pre>';
}
```

So it should be?
MS SQL Server fix for older versions than 2012 (alternative solution)

This alternative solution LIMIT, OFFSET problem SQL Server version older than 2012.
fix error

fix error when if fields whose names are reserved word
Update QueryBuilder.php

fix error occurs when you use multiple databases.
Update QueryBuilder.php

solution used **heukirne** for the names of columns.
Fixes #3716: `DynamicModel::validateData()` does not call `validate()` if the `$rules` parameter is empty
start
Update intro-yii.md
половина готова
Update intro-yii.md
Gii codestyle fixes
guide WIP [skip ci]
Fix case for ActiveField::checkboxTemplate use
Merge pull request #3731 from stevepacker/patch-1

Fix case for ActiveField::checkboxTemplate use
guide WIP [skip ci]
Merge branch 'master' of git://github.com/yiisoft/yii2
Additional `Application` property for `$formatter`
Update start-installation.md

Остался последний раздел
fix typo
Merge pull request #3735 from Ryadnov/patch-2

fix typo