Add a README

DataLad explanation is based on the instructions used for Studyforrest
datasets (shortened).

README.md

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+# SFB 1451 superdataset
+
+This repository is a [DataLad](https://www.datalad.org/) dataset. It
+provides fine-grained data access down to the level of individual
+files, and allows for tracking future updates. In order to use this
+repository for data retrieval, [DataLad](https://www.datalad.org/) is
+required. It is a free and open source command line tool, available
+for all major operating systems, and builds up on Git and
+[git-annex](https://git-annex.branchable.com/) to allow sharing,
+synchronizing, and version controlling collections of large files. You
+can find information on how to install DataLad at
+[handbook.datalad.org/en/latest/intro/installation.html](http://handbook.datalad.org/en/latest/intro/installation.html).
+
+## How to obtain (sub)datasets
+
+A DataLad dataset can be `cloned` by running
+
+```
+datalad clone <url>
+```
+
+Once a dataset is cloned, it is a light-weight directory on your local
+machine.  At this point, it contains only small metadata and
+information on the identity of the files in the dataset, but not
+actual *content* of the (sometimes large) data files.
+
+DataLad datasets can contain other datasets, so called *subdatasets*.
+If you clone the top-level dataset, subdatasets do not yet contain
+metadata and information on the identity of files, but appear to be
+empty directories.
+
+The SFB 1451 superdataset contains one subdataset per project, and
+these can have further subdatasets. In order to retrieve file
+availability metadata in subdatasets, run
+
+```
+datalad get -n <path/to/subdataset>
+```
+
+After cloning a (sub)dataset, you can retrieve file contents, if they
+are stored in a location available to you, by running:
+
+```
+datalad get <path/to/directory/or/file>
+```