Update issue templates

.github/ISSUE_TEMPLATE/custom.md

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+---
+name: Custom issue template
+about: Describe this issue template's purpose here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+### Issue description
+
+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.
+
+Do not report bugs (crashes, incorrect behaviour) without reproducing on a tmux
+built from the latest code in Git.
+
+### Required information
+
+Please provide the following information:
+
+* tmux version (`tmux -V`).
+* Platform (`uname -sp`).
+* $TERM inside and outside of tmux (`echo $TERM`).
+* Logs from tmux (`tmux kill-server; tmux -vv new`).