Add contributors to readme

[michaelbynum]

Summary/Motivation:

This PR adds a list of contributors along with their main contributions to the README. I'll need help getting the list of contributions correct. We should also consider adding more people to the list of contributors.

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[carldlaird]

Looks good to me.