Readme

[temospena]

This file needs to be expanded with a simplified version of the key functions.

[gmatosferreira]

@temospena, I added a chapter on the key functions. What do you think of it?

[gmatosferreira]

@temospena, I updated the README to include a more detailed explanation of the functionality of the package, with excerpts from the several vignettes.

[gmatosferreira]

Check https://ipeagit.github.io/r5r/ for inspiration