Overhaul docs for Windows filemon install

sheesh, hopefully this is "most" of it..although definitely still not
all we need yet..

docs/base/install/windows/rattail.rst

@@ -0,0 +1,20 @@
+
+=========
+ Rattail
+=========
+
+See :doc:`/base/install/common` for more background but if you're just after the
+filemon service then you probably only need to do this:
+
+.. code-block:: sh
+
+   pip install rattail
+
+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.