Add machine-wide pip.ini notes for Windows install

docs/base/install/windows/rattail.rst

@@ -15,6 +15,33 @@ See also :ref:`rattail-sanity-check`.
 Note that the above does not register the filemon service.  That comes
 later; see :doc:`filemon/index`.
 
-You may also need to install additional Rattail packages, e.g. to
-integrate with your POS system.  That is outside of our scope for now
-though.
+
+Additional Packages
+-------------------
+
+In addition to Rattail itself you may need to install other packages.
+(The only known example as of writing would be the LOC SMS integration
+packages, but would of course depend on what you're needing to
+accomplish.)
+
+If the packages exist in the official Python Package Index then you
+can just ``pip install`` them same as for ``rattail`` (above).
+
+However if the packages are in a custom package index then you
+probably want to convey that via machine-wide ``pip.ini`` file.  On
+Windows this usually is at ``C:\ProgramData\pip\pip.ini`` - note that
+you probably have to create the folder as well as the file.
+
+For instance to add Rattail's custom package index, as well as one of
+your own, you might add:
+
+.. code-block:: ini
+
+   [global]
+   extra-index-url =
+       https://pypi.rattailproject.org/simple/
+       https://myusername:mypassword@pypi.example.com/simple/
+
+With such config then you can just ``pip install`` the packages like
+normal, and Pip will consult all package indexes (including official
+PyPI) to find them.