Add community guidelines

[peanutfun]

Community guidelines are a review criterion of JOSS publications. Please add information to this repository on how to seek support, report issues, and how to contribute. This can be supplied in sections of the README.md and in a separate CONTRIBUTING.md file. More information on this topic can be found, e.g., in the Github docs: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

openjournals/joss-reviews#4129

[cgrudz]

Community guidelines are now included in the main documentation:

https://cgrudz.github.io/DataAssimilationBenchmarks.jl/dev/community/Contributing/

As well as the code of conduct:

https://github.com/cgrudz/DataAssimilationBenchmarks.jl/blob/master/CODE_OF_CONDUCT.md