Add `composer install` to contributing documentation
Add `composer install` to contributing documentation

Summary:
Added a description to the CONTRIBUTING.md
how to install required dependencies by composer.

Reviewers: #forgetit-dev, claus.due

Reviewed By: #forgetit-dev, claus.due

Differential Revision: https://phabricator.dkd.de/D73

dkd/php-cmis-client