Added new resource for R language (#6005)

* Added new resource for R language

* Update free-programming-books-langs.md

books/free-programming-books-langs.md

@@ -1941,6 +1941,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 ### R
 
 * [Advanced R Programming](http://adv-r.had.co.nz) - Hadley Wickham
+* [An Introduction to R](https://cran.r-project.org/doc/manuals/R-intro.html) -David M. Smith and William N. Venables
 * [An Introduction to Statistical Learning with Applications in R](https://web.stanford.edu/~hastie/ISLR2/ISLRv2_website.pdf) - Gareth James, Daniela Witten, Trevor Hastie and Robert Tibshirani (PDF)
 * [Cookbook for R](http://www.cookbook-r.com) - Winston Chang
 * [Data Analysis and Prediction Algorithms with R](https://rafalab.github.io/dsbook/) - Rafael A. Irizarry