Added Git/Github book (#3907)

* Added Git/Github book

* alphabetize

* resolve a conflict - resource moved to cheatsheets

* trailing slash
Co-authored-by: NEric Hellman <eric@hellman.net>

free-programming-books.md

@@ -1406,6 +1406,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Git Reference](https://web.archive.org/web/20170602211147/http://gitref.org/)
 * [Git Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/git) (PDF, Kindle) (email address *requested*, not required)
 * [Git Workflows](http://documentup.com/skwp/git-workflows-book)
+* [Happy Git and GitHub for the useR](https://happygitwithr.com) - Jenny Bryan
 * [How to Collaborate on Github](https://github.com/eonist/How-to-collaborate-on-github) - André J
 * [Intoduction to Git and Github](https://launchschool.com/books/git) - Launch School
 * [Intoduction to Git and Github - Tutorial](http://cse.unl.edu/~cbourke/gitTutorial.pdf) - Dr. Chris Bourke (PDF)