github: add Code of Conduct

CODE_OF_CONDUCT.md

+RIOT-OS Code of Conduct
+=======================
+
+RIOT, like so many other free and open source software projects, is made up of a
+mixed group of professionals and volunteers from all over the world, working on
+every aspect of our goals - including teaching, steering, and connecting people.
+
+We want diversity to be one of our greatest strengths, but it can also lead to
+communication issues and unhappiness. This is why we ask people to adhere to a
+few ground rules. They apply equally to founders, maintainers, contributors and
+those seeking help and guidance.
+
+This is not meant to be an exhaustive list of things you are not allowed to do.
+We rather would like you to think of it as a guide to enrich our community and
+the technical community in general with new knowledge and perspectives by
+allowing everyone to participate.
+
+This code of conduct applies to all spaces managed by the RIOT community. This
+includes IRC, the mailing lists, our GitHub projects, face to face events, and
+any other forums created by the community for communication within the
+community. In addition, violations of this code outside these spaces may also
+affect a person's ability to participate within them.
+
+If you believe someone is violating the code of conduct, we ask that you report
+it by emailing <conduct@riot-os.org>. For more details please see our [Reporting
+Guidelines](https://github.com/RIOT-OS/RIOT/wiki/CoC:-Reporting-Guidelines).
+
+-   **Be friendly and patient.**
+-   **Be welcoming.** We strive to be a community that welcomes and supports
+    people of all backgrounds and identities. This includes, but is not limited
+    to members of any race, ethnicity, culture, national origin, colour,
+    immigration status, social and economic class, educational level, sex,
+    sexual orientation, gender identity and expression, age, size, family
+    status, political belief, religion, and mental and physical ability.
+-   **Be considerate.** Your work will be used by other people, and you in turn
+    will depend on the work of others. Any decision you take will affect users
+    and colleagues, and you should take those consequences into account when
+    making decisions. Remember that we're a world-wide community, so you might
+    not be communicating in someone else's primary language.
+-   **Be respectful.** Not all of us will agree all the time, but disagreement
+    is no excuse for poor behavior and poor manners. We might all experience
+    some frustration now and then, but we cannot allow that frustration to turn
+    into a personal attack. It’s important to remember that a community where
+    people feel uncomfortable or threatened is not a productive one. Members of
+    the RIOT community should be respectful when dealing with other members as
+    well as with people outside the RIOT community.
+-   **Be careful in the words that you choose.** We are a community of
+    professionals, and we conduct ourselves professionally. Be kind to others.
+    Do not insult or put down other participants. Harassment and other
+    exclusionary behavior aren't acceptable. This includes, but is not limited
+    to:
+    -   Violent threats or language directed against another person.
+    -   Discriminatory jokes and language.
+    -   Posting sexually explicit or violent material.
+    -   Posting (or threatening to post) other people's personally identifying
+        information ("doxing").
+    -   Personal insults, especially those using racist or sexist terms.
+    -   Unwelcome sexual attention.
+    -   Advocating for, or encouraging, any of the above behavior.
+    -   Repeated harassment of others. In general, if someone asks you to stop,
+        then stop.
+-   **When we disagree, try to understand why.** Disagreements, both social and
+    technical, happen all the time and RIOT is no exception. It is important
+    that we resolve disagreements and differing views constructively. Remember
+    that we’re different. The strength of RIOT comes from its varied community,
+    people from a wide range of backgrounds. Different people have different
+    perspectives on issues. Being unable to understand why someone holds a
+    viewpoint doesn’t mean that they’re wrong. Don’t forget that it is human to
+    err and blaming each other doesn’t get us anywhere. Instead, focus on
+    helping to resolve issues and learning from mistakes.
+
+Text based on the Code of Conduct of the [Django
+community](https://www.djangoproject.com/conduct/).
+
+Questions?
+----------
+
+If you have questions, please see [the FAQ](https://github.com/RIOT-OS/RIOT/wiki/CoC:-FAQ).
+If that doesn't answer your questions, feel free to [contact us](mailto:conduct@riot-os.org).