Add a note referring to the installation of the translation library

AUTHENTICATION.md

@@ -48,6 +48,11 @@ $HOME/.config/gcloud/application_default_credentials.json
 Once we have the `application_default_credentials.json` that we created on the previous step now we
 can instantiate a client which internally using the Google Auth library will take that file and use
 it to authenticate your requests:
+
+**Note**: You can install the Google Translate client library by running `composer install google/cloud-translate`.
+See the [README.md](README.md) for further instructions, or browse
+[the complete list of packages](https://cloud.google.com/php/docs/reference/).
+
 ```php
 require_once 'vendor/autoload.php';