Add Composer support

[Composer](http://getcomposer.org) is a package management system for PHP. Having a `composer.json` available will automatically provide managing React in Composer-based projects.
> What does this mean? I don't see that listed in the [schema](https://getcomposer.org/doc/04-schema.md#type)

*Types* allow you to define how the package should be handled by custom installation logic. If an installer is found for the `type`, it will use it, otherwise it will just use the `library` installer (the default). Using `component` as the type will allow the package to be installed in a directory where web assets should be installed via the Component Installer.

This is the approach taken for [github.com/components](https://github.com/components), as well as [many other packages on Packagist](https://packagist.org/search/?type=component).