Link to online rtd documentation

• adds direct link to online rtd documentation

README.rst

@@ -34,4 +34,9 @@ If you intend to develop code in the toolbox use the -e flag for installation. T
 
 .. code:: bash
 
-    pip install --user -e .
+    pip install --user -e .
\ No newline at end of file
+
+Documentation
+#############
+
+Online documentation can be found at `ReadTheDocs <https://rtd.xfel.eu/docs/scs-toolbox/en/latest/>`_.