`vendor` directory added to the generated application.

I'm proposing to do that. Can't say for other developers, but almost in every new project the first thing i'm doing is creation of the `vendor` directory. `vendor` directory already mentioned in the guide, API and even extensions.

Don't confuse `vendor` with `extensions`. It's different directories. The former is for stuff, like ZF or Symfony2 components without extensions wrappers, while the latter is for pure Yii extensions.