Mention CHANGES.

.github/CONTRIBUTING.md

@@ -2,13 +2,20 @@
 
 Before opening an issue, please ensure that:
 
+- Your problem is a specific problem or question or suggestion, not a general
+  complaint.
+
 - `$TERM` inside tmux is screen, screen-256color, tmux or tmux-256color. Check
   by running `echo $TERM` inside tmux.
 
 - You can reproduce the problem with the latest tmux release, or a build from
   Git master.
 
-- Your question or issue is not covered in the manual (run man tmux).
+- Your question or issue is not covered [in the
+  manual](https://man.openbsd.org/tmux.1) (run `man tmux`).
+
+- Your problem is not mentioned in the [CHANGES
+  file](https://raw.githubusercontent.com/tmux/tmux/master/CHANGES) file.
 
 - Nobody else has opened the same issue recently.
 

.github/ISSUE_TEMPLATE.md

@@ -3,6 +3,9 @@
 Please read https://github.com/tmux/tmux/blob/master/.github/CONTRIBUTING.md
 before opening an issue.
 
+If you have upgraded, make sure your issue is not covered in the CHANGES file
+for your version: https://raw.githubusercontent.com/tmux/tmux/master/CHANGES
+
 Describe the problem and the steps to reproduce. Add a minimal tmux config if
 necessary. Screenshots can be helpful, but no more than one or two.