Add link to 6th edition README

Author: mhartl

README.md

@@ -5,6 +5,8 @@ This is the sample application for the
 Learn Web Development with Rails*](https://www.railstutorial.org/)
 by [Michael Hartl](https://www.michaelhartl.com/).
 
+See also the [6th edition README](https://github.com/learnenough/sample_app_6th_ed#readme).
+
 ## License
 
 All source code in the [Ruby on Rails Tutorial](https://www.railstutorial.org/)