Replaced relative link by absolute URL in README.rst to make the README.rst work on PyPI.

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

README.rst

@@ -74,10 +74,11 @@ homogenization jobs (using gms_preprocessing) can be defined and started. All
 configuration parameters of gms_preprocessing are accessible in the UI.
 
 
-.. image:: ./docs/images/webapp_screenshot_900x497.png
+.. image:: https://gitext.gfz-potsdam.de/geomultisens/gms_preprocessing/-/blob/master/docs/images/webapp_screenshot_900x497.png
     :width: 900 px
     :height: 497 px
     :scale: 100 %
+    :alt: WebApp Screenshot
 
 
 Usage via console interface