add a .cff file to the main renku repo

[rokroskar]

As suggested in a discourse comment we should add a cff file to the main renku repo to make it clear how renku could be cited. There are a few issues here - first, we don't get dois for renku. Second, there isn't a paper describing renku. So part of this issue is to figure out what the current accepted best-practice is for software citations in absence of a doi or publication.