Added Code of Conduct

CODE_OF_CONDUCT.md

+# TNL Code of Conduct
+
+The TNL project welcomes and encourages participation by everyone. Therefore, we have adopted this
+code of conduct in order to create an open and welcoming environment for everyone, regardless of
+age, body size, disability, ethnicity, gender, level of experience, education, nationality, race,
+religion, or sexual identity and orientation. No matter how you identify yourself or how others
+perceive you, you are welcome in our community.
+
+## Our standards
+
+1. __Be respectful.__
+
+   Disagreements are inevitable among people in a large and diverse community. Remain respectful of
+   differing viewpoints and experiences. Disagreement is no excuse for poor behaviour or personal
+   attacks.
+
+2. __Assume good faith.__
+
+   There are many ways to reach the goal of the project, which may differ from your ways. Assume
+   that other people are working towards this goal. Keep in mind that the people you communicate
+   with may not be native English speakers or may have different cultural backgrounds. Be careful
+   in the words that you choose to avoid conflicts and misapprehension.
+
+3. __Be inquisitive.__
+
+   TNL is a large and complex project where nobody knows everything. It is always good to ask
+   questions early to avoid problems later. Similarly, offers for help are equally important.
+
+4. __Be collaborative.__
+
+   TNL is built on the work done in other projects and it is in turn used by further projects. When
+   you contribute to TNL, be willing to explain to others how it works, so that they can build on
+   your work to make it even better.
+
+5. __Try to be concise.__
+
+   Keep in mind that what you write once will be read by many other persons. Try to use a clear and
+   concise language, especially when a long explanation is necessary. Try to stay on topic, so that
+   the whole discussion can be followed as efficiently as possible.
+
+## In case of problems
+
+While this code of conduct should be adhered to by participants, we recognize that sometimes people
+may have a bad day, or be unaware of some of the guidelines in this code of conduct. When that
+happens, you may reply to them and point out this code of conduct. However, it should still adhere
+to the relevant parts of this code of conduct; in particular, it should not be abusive or
+disrespectful. Assume good faith; it is more likely that participants are unaware of their bad
+behaviour than that they intentionally try to degrade the quality of the discussion.
+
+Serious or persistent offenders will be temporarily or permanently banned from participating in the
+TNL community. Complaints should be made to the maintainers of the TNL project. To find contact
+information for these administrators, please see the [project's website](https://tnl-project.org/).
+
+## Attribution
+
+This document has been adapted from several sources, notably the
+[Debian Code of Conduct version 1.0](https://www.debian.org/code_of_conduct)
+and the [SciPy Code of Conduct](https://scipy.github.io/devdocs/dev/conduct/code_of_conduct.html).