add README for data

Signed-off-by: Maximilian Huber <[email protected]>

data/README.md

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+# data
+
+This directory verndors data from other sources.
+They are all under their respective licenses as described in their directories.
+They are included as `git subtree`s to not be annoyed by submodules and to capture their complete history here.
+
+## Updating
+
+there are several `update.*` scripts in `./data` that can be used to update the sources. And there is a `all_update.sh` to run them all.