Add a 'Support' section to the README file

I added a new 'Support' section to the README so that we could have links to the issue queue, Drupal Answers, and search links to old issues on drupal.org all in one place.

I also moved the pointer to CONTRIBUTING.md into this section.

Neat GitHub feature:  you can see this live here:  https://github.com/greg-1-anderson/drush/tree/readme-support