autoloader draft docs

docs/autoloader.md

+Yii2 class loader
+=================
+
+Yii 2 class loader is PSR-0 compliant. That means it can handle most of the PHP
+libraries and frameworks out there.
+
+In order to autoload a library you need to set a root alias for it.
+
+PEAR-style libraries
+--------------------
+
+```php
+\Yii::setAlias('@Twig', '@app/vendors/Twig');
+```
+
+References
+----------
+
+- YiiBase::autoload
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