Update CONTRIBUTING.md

The PHPUnit tests run into error messages if PHPWord has not been installed by Composer.

Author: Franz Holzinger

CONTRIBUTING.md

@@ -6,7 +6,7 @@ We want to create a high quality document writer and reader library that people
 
 - **Be brief, but be bold**: State your issues briefly. But speak out your ideas loudly, even if you can't or don't know how to implement it right away. The world will be better with limitless innovations.
 - **Code in PSR**: We use PHP Standards Recommendation (PSR) from the PHP Framework Interoperability Group (PHP-FIG). If you're not familiar with this standard, please [familiarize yourself now](https://github.com/php-fig/fig-standards). Use [PHPCodeSniffer](http://pear.php.net/package/PHP_CodeSniffer/) to check your code against this standard.
-- **Test your code**: Nobody else knows your code better than you. So please, test your own code before submitting your pull request. PHPWord uses [PHPUnit](http://phpunit.de/) as our testing framework.
+- **Test your code**: Nobody else knows your code better than you. So please, test your own code before submitting your pull request. PHPWord uses [PHPUnit](http://phpunit.de/) as our testing framework. The PHPUnit tests only work if PHPWord has been fetched and installed by Composer.
 - **Request pull in separate branch**: Do not submit your request to the master branch. But create a separate branch named specifically for the issue that you addressed. Read [GitHub manual](https://help.github.com/articles/using-pull-requests) to find out more about this. If you are new to GitHub, read [this short manual](https://help.github.com/articles/fork-a-repo) to get yourself familiar with forks and how git works in general.
 
 That's it. Thank you for your interest in PHPWord, and welcome!