Update README

[mcmancini]

Update README with the following:

• Brief description of the functionality of the package
• Authors, affiliation and licence
• Table of contents of the README
• Instructions for installation
• Instructions on how to use the package
• Instructions on how to run tests (if done)
• References and outside sources

We will populate and update this as we build the package. Keep the issue open and create pull requests any time an update, even if partial, has been made.