Improve contributing guidelines for composer newbies

[m7913d]

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'.

This is:

• [ ] a bugfix
• [ ] a new feature
• [ ] refactoring
• [ ] additional unit tests
• [x] documentation improvement

Checklist:

• [ ] Changes are covered by unit tests
  • [ ] Changes are covered by existing unit tests
  • [ ] New unit tests have been added
• [x] Code style is respected
• [x] Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• [ ] CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• [x] Documentation is updated as necessary

Why this change is needed?

This PR helps people unfamiliar with composer to contribute to this project.

[oleibman]

Thank you for your contribution.