Add how to test suggestion to code review guidelines

doc/development/code_review.md

@@ -281,6 +281,8 @@ first time.
   of your shiny new branch, read through the entire diff. Does it make sense?
   Did you include something unrelated to the overall purpose of the changes? Did
   you forget to remove any debugging code?
+- Consider providing instructions on how to test the merge request. This can be
+  helpful for reviewers not familair with the product feature or area of the codebase.
 <!-- vale gitlab.FutureTense = NO -->
 - Be grateful for the reviewer's suggestions. ("Good call. I'll make that
   change.")