offlineimap.conf: mbnames: provide sample for the folderfilter option

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -220,6 +220,19 @@ accounts = Test
 # This filter can be used only to further restrict mbnames to a subset of
 # folders that pass the account's folderfilter.
 #
+# E.g.: with mbnames_folderfilter defined like this in the python file:
+#
+# def mbnames_folderfilter(accountname, foldername):
+#     allowed = {'myaccount': ['folderA', 'folderB']}
+#     if accountname in allowed:
+#         return foldername in allowed[accountname]
+#     return False
+#
+#folderfilter = mbnames_folderfilter
+
+
+# This option stands in the [mbnames] section.
+#
 # You can customize the order in which mailbox names are listed in the generated
 # file by specifying a sort_keyfunc, which takes a single dict argument
 # containing keys 'accountname' and 'foldername'.  This function will be called