Added R programming resource for program evaluation - Summary and Analysis of...

Added R programming resource for program evaluation - Summary and Analysis of Extension Program Evaluation in R (#4794)

check_urls=free-programming-books.md

free-programming-books.md

@@ -2461,6 +2461,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [R Programming](https://en.wikibooks.org/wiki/R_Programming) - Wikibooks
 * [R Programming for Data Science](https://leanpub.com/rprogramming) - Roger D. Peng  *(Leanpub account or valid email requested)*
 * [R Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/rsuccinctly) (PDF, Kindle) (email address *requested*, not required)
+* [Summary and Analysis of Extension Program Evaluation in R](https://rcompanion.org/handbook/index.html) - Salvatore S. Mangiafico
 * [The caret Package](http://topepo.github.io/caret/index.html) - Max Kuhn
 * [The R Inferno](http://www.burns-stat.com/pages/Tutor/R_inferno.pdf) - Patrick Burns (PDF)
 * [The R Language](http://stat.ethz.ch/R-manual/R-patched/doc/html)