Update README with details

[elenagan]

• [x] a summary paragraph that describes the project at a high level

• [x] a bulleted list of the functions (and datasets if applicable) that will be included in the package (this should be a 1-2 sentence description for each function/dataset)

• [x] a paragraph describing where your packages fit into the R ecosystem (are there any other R packages that have the same/similar functionality? Provide links to any that do. If none exist, then clearly state this as well).

[mikeguron]

Updated README file with all the sections as outlined above