Update README.md

Author: rmaestre

README.md

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-# additive-splines-regression-
-Spline regression and additive models (Simple introduction with C++). We provide an example of C++ implementation of additive regression splines through Rcpp and RcppArmadillo 
+# Additive splines regression
+Spline regression and additive models (Simple introduction with C++). We provide an example of C++ implementation of additive regression splines (univariate, univariate penalized and bivariate penalized) through Rcpp and RcppArmadillo.
+
+Main documents are
+1. [The workbook with examples](https://github.com/bbvadata/additiveRegressionSpline/blob/master/rmd/workbook.pdf)
+
+2. [The C++ code through Rcpp](https://github.com/bbvadata/additiveRegressionSpline/blob/master/src/gam.cpp)
+
+2. [The R code provided [1]](https://github.com/bbvadata/additiveRegressionSpline/blob/master/src/gam.cpp)
+
+[1] [Generalized Additive Models: An Introduction with R](https://www.crcpress.com/Generalized-Additive-Models-An-Introduction-with-R/Wood/p/book/9781584884743)