add a CONTRIBUTING file

See https://github.com/blog/1184-contributing-guidelines

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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+# i3status/i3lock bugreports/feature requests
+
+Note that i3status and i3lock related bugreports and feature requests should be
+filed in the corresponding repositories, i.e. https://github.com/i3/i3status
+and https://github.com/i3/i3lock
+
+# i3 bugreports/feature requests
+
+1. Read http://i3wm.org/docs/debugging.html
+2. Make sure you include a link to your logfile in your report (section 3).
+3. Make sure you include the i3 version number in your report (section 1).
+
+# Pull requests
+
+* Before sending a pull request for new features, please check with us that the
+  feature is something we want to see in i3 by opening an issue which has
+  “feature request” or “enhancement” in its title.
+* Use `clang-format` to format your code.
+* Run the testsuite, see http://i3wm.org/docs/testsuite.html