Added JavaScript for Rails Developers to books (#491)

Author: eelcoj

app/content/data/books.yml

@@ -30,3 +30,6 @@
 
 - name: "Master Hotwire: Build Modern Web Apps with Rails Simplicity"
   url: https://masterhotwire.com/?utm_source=hotwireweekly&utm_medium=email&utm_campaign=week-51-hotwire-native-1-1-hotwire-spark
+
+- name: "JavaScript for Rails Developers"
+  url: https://javascriptforrails.com/