Add zenodo badge

Author: Birgit Schachler

README.rst

@@ -2,6 +2,8 @@
     :target: https://travis-ci.org/wind-python/windpowerlib
 .. image:: https://coveralls.io/repos/github/wind-python/windpowerlib/badge.svg?branch=dev
     :target: https://coveralls.io/github/wind-python/windpowerlib?branch=dev
+.. image:: https://zenodo.org/badge/DOI/10.5281/zenodo.824268.svg
+   :target: https://doi.org/10.5281/zenodo.824268
 
 Introduction
 =============
@@ -122,4 +124,4 @@ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.
 
 You should have received a copy of the GNU General Public License
-along with this program.  If not, see http://www.gnu.org/licenses/.
+along with this program.  If not, see http://www.gnu.org/licenses/.