Add usability details to README

[higgi13425]

There are two data files available. These are common.rdata and vehicles.rdata. The common dataset contains make, model, n, and years that particular make and model was available between 1984-2015, inclusive, for 347 car models. The vehicles dataset contains 33,442 entries on id, make, model, year, vehicle class, transmission, drive type, cylinders, displacement, fuel type, and highway and city mileage ratings per the EPA. After installing the package, you can access the data files with data("vehicles") or data("common") these will result in {Promise}s appearing in your Global Environment. To actually access the data, you have to run a command like head(vehicles) or head(common). These will result in the dataframe appearing in your Environment tab.