Add 'next steps' info to Composer installation output

Problem/Motivation

In Slack chat, @mixologic mentions that when you use Composer to build a Symfony project, you get some output at the very end helping you move forward with the next step.

Proposed resolution

Add a 'next steps' summary to the end of the composer install process for Drupal.

This would probably be a separate plugin, so that you can turn it off using composer remove. In fact, telling you how to do that will be one of the next steps. :-)

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet