[Tei-council] release notes

I updated the release notes *in the release-4.1.0 branch* (because that's what Martina did). While there are always improvements to be made, I think they are mostly good to go. Still need a release code name (why do we have those -- they are fun for us, but since we do not actually use them ...?). And I think we should consider deleting the prhase "to represent primary source materials" from the 1st bullet point.