Include composer dependencies in repo

vendor/symfony/polyfill-mbstring/README.md

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+Symfony Polyfill / Mbstring
+===========================
+
+This component provides a partial, native PHP implementation for the
+[Mbstring](http://php.net/mbstring) extension.
+
+More information can be found in the
+[main Polyfill README](https://github.com/symfony/polyfill/blob/master/README.md).
+
+License
+=======
+
+This library is released under the [MIT license](LICENSE).