add code of conduct

- [x] Is it mergable?
- [x] Did it pass the tests?
- N/A If it introduces new functionality in scripts/ is it tested?
  Check for code coverage.
- [x] Is it well formatted? Look at `make pep8`, `make diff_pylint_report`,
  `make cppcheck`, and `make doc` output. Use `make format` and manual fixing as needed.
- [x] Is it documented in the ChangeLog?
- [x] Was a spellchecker run on the source code and documentation after
  changes were made?
Would suggest "By participating in the development of this project you agree to abide by its terms."