add cdsapi installation instructions

scripts/build_cutout.py

@@ -1,6 +1,15 @@
 """
 Create cutouts with `atlite <https://atlite.readthedocs.io/en/latest/>`_.
 
+For this rule to work you must have
+
+- installed the `Copernicus Climate Data Store <https://cds.climate.copernicus.eu>`_ ``cdsapi`` package  (`install with `pip``) and
+- registered and setup your CDS API key as described `on their website <https://cds.climate.copernicus.eu/api-how-to>`_.
+
+.. seealso::
+    For details on the weather data read the `atlite documentation <https://atlite.readthedocs.io/en/latest/>`_.
+    If you need help specifically for creating cutouts `the corresponding section in the atlite documentation <https://atlite.readthedocs.io/en/latest/examples/create_cutout.html>`_ should be helpful.
+
 Relevant Settings
 -----------------
 
@@ -73,9 +82,6 @@ A **SARAH-2 cutout** can be used to amend the fields ``temperature``, ``influx_t
     .. image:: ../img/sarah.png
         :scale: 40 %
 
-.. seealso::
-    For details on the weather data read the `atlite documentation <https://atlite.readthedocs.io/en/latest/>`_.
-
 Description
 -----------