Exclude top-level files from annex
This configuration explicitly states that the top-level descriptive
files (datacite, license, readme) should not be annexed, and should help
avoiding accidentally annexing them in future commits.
There are many more files in this dataset that were committed to git.
Since the dataset seems to be mainly used for data deposition and does
not undergo active changes, it seems that there is no need to
(retroactively) write rules for files that are stored beyond the top
level directory.
