Add a simple code of conduct based on ruby community guidelines

This was discussed at the systemd hackfest during ASG2018, and
we agreed to use the Ruby text [1] with the enforcement clause based on
the "contributor covenant". I obviously modified the text where applicable
to refer to systemd.

[1] https://www.ruby-lang.org/en/conduct/

Fixes 10148.

docs/code_of_conduct.md

@@ -0,0 +1,14 @@
+# The Systemd Community Conduct Guidelines
+
+This document provides community guidelines for a safe, respectful, productive, and collaborative place for any person who is willing to contribute to systemd. It applies to all “collaborative spaces”, which is defined as community communications channels (such as mailing lists, submitted patches, commit comments, etc.).
+
+- Participants will be tolerant of opposing views.
+- Participants must ensure that their language and actions are free of personal attacks and disparaging personal remarks.
+- When interpreting the words and actions of others, participants should always assume good intentions.
+- Behaviour which can be reasonably considered harassment will not be tolerated.
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting the project team at systemd-conduct@googlegroups.com. This team currently consists of David Strauss <<david@davidstrauss.net>>, Ekaterina Gerasimova (Kat) <<Kittykat3756@gmail.com>>, and Zbigniew Jędrzejewski-Szmek <<zbyszek@in.waw.pl>>. It is also possible to contact any member directly.
+
+All complaints will be reviewed and investigated and will result in a response that is deemed necessary and appropriate to the circumstances. The project team is obligated to maintain confidentiality with regard to the reporter of an incident.