Add a CONTRIBUTING file with some information on opening issues.

CONTRIBUTING

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+When reporting issues:
+
+YOU MUST INCLUDE THE TMUX VERSION
+
+DO NOT OPEN AN ISSUE THAT DOES NOT MENTION THE TMUX VERSION
+
+Please 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.
+
+Please do not report bugs (crashes, incorrect behaviour) without reproducing on
+a tmux built from Git master.
+
+To run tmux without a config and get logs, run:
+
+    tmux -Ltest kill-server
+    tmux -vvvv -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.