Add tools to modules.rst

Author: SabineHaas

doc/modules.rst

@@ -93,4 +93,15 @@ Methods of the ModelChain object.
 
    modelchain.ModelChain.rho_hub
    modelchain.ModelChain.v_wind_hub
-   modelchain.ModelChain.turbine_power_output
+   modelchain.ModelChain.turbine_power_output
+
+
+Tools
+==============
+
+Additional functions used in the windpowerlib.
+
+.. autosummary::
+   :toctree: temp/
+
+   tools.select_closer_value