[doc]Composer is not a package manager.

It is a issue i found when I translate the composer.md file.
__Composer is not a package manager, It's a tool for dependency management.__

According to their [doc](https://getcomposer.org/doc/00-intro.md)
> **Composer is a tool for dependency management in PHP.** It allows you to declare the dependent libraries your project needs and it will install them in your project for you.

and 

> **Composer is not a package manager.** Yes, it deals with "packages" or libraries, but it manages > them on a per-project basis, installing them in a directory (e.g. vendor) inside your project. By > default it will never install anything globally. Thus, it is a dependency manager.

So i think, It would be better, if we remain their offical appellation. What do you think?