Documentation: explain dummyprocess

The dummyprocess module can be used to run a single
shell command; it can be used effectively with instances
to run one or more commands during installation.

settings.conf

@@ -37,6 +37,10 @@ modules-search: [ local ]
 # For more information on running module instances, run Calamares in debug
 # mode and check the Modules page in the Debug information interface.
 #
+# A module that is often used with instances is dummyprocess, which will
+# run a single (shell) command. By configuring more than one instance of
+# the module, multiple shell commands can be run during install.
+#
 # YAML: list of maps of string:string key-value pairs.
 #instances:
 #- id:       owncloud