Update CONTRIBUTING.md, mostly from Nicolas CARPi.

.github/CONTRIBUTING.md

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+# What should I do before opening an issue?
+
+Before opening an issue, please ensure that:
+
+- 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).
+
+- Nobody else has opened the same issue recently.
+
+# What should I include in an isue?
+
+Please include the output of:
+
+~~~bash
+uname -sp && tmux -V && echo $TERM
+~~~
+
+Also include:
+
+- Your platform (Linux, OS X, or whatever).
+
+- A brief description of the problem with steps to reproduce.
+
+- A minimal tmux config, if you can't reproduce without a config.
+
+- Your terminal, and $TERM inside and outside of tmux.
+
+- Logs from tmux (see below).
+
+- At most one or two screenshots, if helpful.
+
+# How do I test without a .tmux.conf?
+
+Run a separate tmux server with -f/dev/null to skip loading .tmux.conf:
+
+~~~bash
+tmux -Ltest kill-server
+tmux -Ltest -f/dev/null new
+~~~
+
+# How do I get logs from tmux?
+
+Add -vv to tmux to create three log files in the current directory. If you can
+reproduce without a configuration file:
+
+~~~bash
+tmux -Ltest kill-server
+tmux -vv -Ltest -f/dev/null new
+~~~
+
+Or if you need your configuration:
+
+~~~base
+tmux kill-server
+tmux -vv new
+~~~
+
+The log files are:
+
+- tmux-server*.log: server log file.
+
+- tmux-client*.log: client log file.
+
+- tmux-out*.log: output log file.
+
+To run tmux without a config and get logs, run:
+
+~~~bash
+tmux -Ltest kill-server
+tmux -vv -Ltest -f/dev/null new
+~~~
+
+Then reproduce the problem, exit tmux, and attach the `tmux-server-*.log` file
+from the current directory to the issue.