[netinstall] Document local URL

- `local` is supposed to read from the config-file, rather than
  externally; this simplifies examples, makes it easier to have
  multiple netinstalls, and condenses the documentation.

src/modules/netinstall/netinstall.conf

@@ -11,7 +11,11 @@
 #
 # The format of the groups file is documented in `README.md`.
 #
-# groupsUrl: file:///usr/share/calamares/netinstall.yaml
+# As a special case, setting *groupsUrl* to the literal string
+# `local` means that the data is obtained from **this** config
+# file, under the key *groups*.
+#
+groupsUrl: local
 
 # If the installation can proceed without netinstall (e.g. the Live CD
 # can create a working installed system, but netinstall is preferred
@@ -46,3 +50,36 @@ label:
     # sidebar[nl]: "Pakketkeuze"
     # title: "Office Package"
     # title[nl]: "Kantoorsoftware"
+
+# If, and only if, *groupsUrl* is set to the literal string `local`,
+# groups data is read from this file. The value of *groups* must be
+# a list, with the same format as the regular `netinstall.yaml` file.
+#
+# This is recommended only for small static package lists.
+groups:
+    - name: "Default"
+      description: "Default group"
+      hidden: true
+      selected: true
+      critical: false
+      packages:
+        - base
+        - chakra-live-skel
+    - name: "Shells"
+      description: "Shells"
+      hidden: false
+      selected: false
+      critical: true
+      subgroups:
+        - name: "Bash"
+          description: "Bourne Again Shell"
+          selected: true
+          packages:
+            - bash
+            - bash-completion
+        - name: "Zsh"
+          description: "Zee shell, boss"
+          packages:
+            - zsh
+            - zsh-completion
+            - zsh-extensions