docs: add documentation about site modules

docs/user/site.rst

@@ -355,6 +355,41 @@ utilities are installed.
    Depending on the context, you might be able to use comments like
    ``<!-- empty -->`` as translations to effectively hide the text.
 
+Site modules
+------------
+
+The file ``modules`` in the site repository is completely optional and can be used
+to supply additional package feeds from which packages are built. The git repositories
+specified here are retrieved in addition to the default feeds when ``make update``
+it called.
+
+This file's format is very similar to the toplevel ``modules`` file of the Gluon
+tree, with the important different that the list of feeds must be assigned to
+the variable ``GLUON_SITE_FEEDS``. Multiple feed names must be separated by spaces,
+for example::
+
+    GLUON_SITE_FEEDS='foo bar'
+
+The feed names may only contain alphanumerical characters, underscores and slashes.
+For each of the feeds, the following variables are used to specify how to update
+the feed:
+
+PACKAGES_${feed}_REPO
+    The URL of the git repository to clone (usually ``git://`` or ``http(s)://``)
+
+PACKAGES_${feed}_COMMIT
+    The commit ID of the repository to use
+
+PACKAGES_${feed}_BRANCH
+    Optional: The branch of the repository the given commit ID can be found in.
+    Defaults to the default branch of the repository (usually ``master``)
+
+These variables are always all uppercase, so for an entry ``foo`` in GLUON_SITE_FEEDS,
+the corresponding configuration variables would be ``PACKAGES_FOO_REPO``,
+``PACKAGES_FOO_COMMIT`` and ``PACKAGES_FOO_BRANCH``. Slashes in feed names are
+replaced by underscores to get valid shell variable identifiers.
+
+
 Examples
 --------