fix(docs): Update CONTRIBUTING.md to reference .php-cs-fixer.dist.php (GoogleCloudPlatform#1681)

Co-authored-by: Saransh Dhingra 

Author: glasnt

CONTRIBUTING.md

@@ -97,7 +97,7 @@ guidelines for all Google Cloud samples.
 [style-guide]: https://googlecloudplatform.github.io/samples-style-guide/
 
 Samples in this repository also follow the [PSR2][psr2] and [PSR4][psr4]
-recommendations. This is enforced using [PHP CS Fixer][php-cs-fixer].
+recommendations. This is enforced using [PHP CS Fixer][php-cs-fixer], using the config in [.php-cs-fixer.dist.php](.php-cs-fixer.dist.php)
 
 Install that by running
 
@@ -108,8 +108,8 @@ composer global require friendsofphp/php-cs-fixer
 Then to fix your directory or file run 
 
 ```
-php-cs-fixer fix .
-php-cs-fixer fix path/to/file
+php-cs-fixer fix . --config .php-cs-fixer.dist.php
+php-cs-fixer fix path/to/file --config .php-cs-fixer.dist.php
 ```
 
 The [DLP snippets](https://github.com/GoogleCloudPlatform/php-docs-samples/tree/master/dlp) are an example of snippets following the latest style guidelines.