docs(pillar): add pillar example for TOFS pattern

pillar.example

     - 192.168.0.1
     - host.example.test
 
-# yamllint disable-line rule:comments-indentation
   # Example for require/require_in/include
   # See: https://github.com/saltstack-formulas/apt-cacher-formula/pull/12 for details
   # apt_cacher_ng:
   #     # custom states: i.e. Debian package repos
   #     - 'pkgrepo: deb jessie-backports'
   #     - 'pkgrepo: deb-src jessie-backports'
+
+  tofs:
+    # The files_switch key serves as a selector for alternative
+    # directories under the formula files directory. See TOFS pattern
+    # doc for more info.
+    # Note: Any value not evaluated by `config.get` will be used literally.
+    # This can be used to set custom paths, as many levels deep as required.
+    # files_switch:
+    #   - any/path/can/be/used/here
+    #   - id
+    #   - roles
+    #   - osfinger
+    #   - os
+    #   - os_family
+    # All aspects of path/file resolution are customisable using the options below.
+    # This is unnecessary in most cases; there are sensible defaults.
+    # Default path: salt://< path_prefix >/< dirs.files >/< dirs.default >
+    #         I.e.: salt://template/files/default
+    # path_prefix: template_alt
+    # dirs:
+    #   files: files_alt
+    #   default: default_alt
+    # The entries under `source_files` are prepended to the default source files
+    # given for the state
+    source_files:
+      apt-cacher/ng/client/config/file:
+        - 'alt_client.conf'
+      apt-cacher/ng/server/config/file:
+        - 'alt_server.conf'
+      apt-cacher/ng/server/config/credentials:
+        - 'alt_security.conf'