docs: reference caching howto in workflow section (#4789)

It might be helpful to people learning about AiiDA workflows to know that caching exists and point them in that direction.

Co-authored-by: Leopold Talirz <leopold.talirz@gmail.com>

docs/source/howto/workflows.rst

@@ -21,6 +21,10 @@ Here we present a brief introduction on how to write both workflow types.
 
     For more details on the concept of a workflow, and the difference between a work function and a work chain, please see the corresponding :ref:`topics section<topics:workflows:concepts>`.
 
+.. note::
+
+   Developing workflows may involve running several lengthy calculations. Consider :ref:`enabling caching <how-to:run-codes:caching>` to help avoid repeating long workflow steps.
+
 Work function
 -------------