Add a .keep file for the relnotes folder

[joseph-wakeling-sociomantic]

This will ensure that the relnotes folder is preserved by neptune when clearing its contents after a release.

[jens-mueller-sociomantic]

Is this something that neptune does or it is more like that this way you ensure that there will be always a file inside relnotes, i.e., git will keep tracking the directory.

[joseph-wakeling-sociomantic]

The latter. It means git will still keep track of the folder when neptune clears out its other contents. I've updated the commit message accordingly.