Add more detail.

python/examples/8-showing_custom_bonds.py

@@ -5,9 +5,13 @@
 This example demonstrates how to add shapes into the chemiscope output such
 that custom bonds that would not normally be assigned by ASE can be rendered.
 
-This is done by using stk to generate and analyse molecules, which comes with
-topology/bonding information by default (using the cheminformatic software
-rdkit).
+This is done by using `stk <https://stk.readthedocs.io/en/stable/>`_ to
+generate and analyse molecules, which comes with topology/bonding information
+by default (using the cheminformatic software rdkit).
+
+We use `stko <https://stko-docs.readthedocs.io/en/latest/>`_ to calculate
+some rudimentary properties of `stk` molecules. `stko` can be installed with
+``pip install stko``.
 
 """