Add community guidelines

[jrfep]

Following examples from other packages, we should add a Code of conduct and a Contributing guide.

We want to welcome contributions and create a community of users and contributors, but also ensure that we interact in a positive and constructive way.

[jrfep]

Add link to the IUCN Red List of Ecosystems Forum as well. Users can participate in the more general forum for questions related to the IUCN RLE, or in GitHub for the more technical discussions about the package. This should also be in the front page of the red-list-ecosystems organisation.