add R Development Guide to gallery

(cherry picked from commit 36ef264)

Author: hturner

docs/gallery/books/books-r-development-guide.png

@@ -435,6 +435,10 @@
       href: https://cleaning-data-r.ala.org.au/
       code: https://github.com/AtlasOfLivingAustralia/cleaning_data/
       thumbnail: books/books-cleaning-bd-in-r.png
+    - title: R Development Guide
+      href: https://contributor.r-project.org/rdevguide/
+      code: https://github.com/r-devel/rdevguide
+      thumbnail: books/books-r-development-guide.png
 
 
 - category: Interactive Docs