Added downloads badge.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>

README.rst

@@ -35,6 +35,8 @@ Status
 .. image:: https://img.shields.io/conda/pn/danschef/py_tools_ds.svg
         :target: https://anaconda.org/danschef/py_tools_ds
         :alt: conda platform
+.. image:: https://img.shields.io/pypi/dm/py_tools_ds.svg
+        :target: https://pypi.python.org/pypi/py_tools_ds
 
 See also the latest coverage_ report and the nosetests_ HTML report.