update doc

docs/building/sources/mars.rst

@@ -1 +1,18 @@
-TODO
+The ``mars`` source will retrieve the data from the ECMWF MARS archive.
+For that, you need to have an ECMWF account and build your dataset on
+one of the Centre's computers, or use the ``ecmwfapi`` Python package.
+
+The `yaml` block can contain any keys that following the `MARS language
+specification`_, with the exception of the ``date``, ``time``` and
+``step``.
+
+The missing keys will be filled with the default values, as defined in
+the MARS language specification.
+
+.. code:: yaml
+
+   mars:
+       levtype: sfc
+       param: [2t, msl]
+
+.. _mars language specification: https://confluence.ecmwf.int/display/UDOC/MARS+user+documentation