Update adding_pkgs.rst

Author: ngam

src/maintainer/adding_pkgs.rst

@@ -641,7 +641,7 @@ installed on your machine.
 You need to define an environment variable named ``CONFIG``. Its value must be
 the name of one of the three YAML configuration files in the ``.ci_support``
 directory (either ``linux64``, ``osx64``, or ``win64``). As an example, you can
-invoke the command as follows.
+invoke the command as follows. For this to work correctly, you must have ``shyaml`` installed on your machine, for example ``conda install -c conda-forge shyaml``.
 
 .. code-block:: sh