Added more specific description on how to add covariates.

[LSmantiz]

Dear Matthieu,

thank you for building a framework to make the RDD framework accessible to many. I came across your package to do a RDD analysis and got stuck in how to add covariates to RDDdata object. As I had to look into the R code to find out how it worked, I added a short description to the function to ease the usage for future users.

Hope that helps. Have a great day. Best Lasare