Improve contributing guidelines for composer newbies

To allow running `composer versions`, it is necessary to call `composer install` to install the (development) dependencies of this project. This may not be obvious to contributors unfamiliar with `composer'.

Author: m7913d

CONTRIBUTING.md

@@ -4,6 +4,7 @@ If you would like to contribute, here are some notes and guidelines:
 
  - All new development should be on feature/fix branches, which are then merged to the `master` branch once stable and approved; so the `master` branch is always the most up-to-date, working code
  - If you are going to submit a pull request, please fork from `master`, and submit your pull request back as a fix/feature branch referencing the GitHub issue number
+ - Install (development) dependencies by running `composer install` inside your PhpSpreadsheet clone.
  - The code must work with all PHP versions that we support.
    - You can call `composer versions` to test version compatibility. 
  - Code style should be maintained.