Adding CONTRIBUTORS.md file for reference

If you have cloned a repository, it is trivial to get the list of
contributors from the repo; Without a repository clone it is
non-trivial.

The CONTRIBUTORS.md file is there as a means of easily promoting and
referencing "These are the people that have contributed to the project."

The initial CONTRIBUTORS.md was created via `./script/contributors.sh`
Merge pull request #8 from projecthydra/adding-contributors

Adding CONTRIBUTORS.md file for reference