Mention composer fix in contributing guide

Author: PowerKiKi

CONTRIBUTING.md

@@ -5,6 +5,7 @@ If you would like to contribute, here are some notes and guidelines:
  - All new development happens on feature/fix branches referenced with the GitHub issue number, and are then merged to the develop branch; so the develop branch is always the most up-to-date, working code
  - The master branch only contains tagged releases
  - If you are going to be submitting a pull request, please fork from develop, and submit your pull request back as a fix/feature branch referencing the GitHub issue number
+ - Code style might be automatically fixed by `composer fix`
  - All code changes must be validated by `composer check`
  - [Helpful article about forking](https://help.github.com/articles/fork-a-repo/ "Forking a GitHub repository")
  - [Helpful article about pull requests](https://help.github.com/articles/using-pull-requests/ "Pull Requests")