doc: Make sure the ruled module init.lua is ignored.

This has to be in its own commit otherwise Travis will fail. This
is because it will keep the file in the build directory when
iterating all commits. Then `ldoc` will fail because the file doesn't
have documentation. If `config.ld` is updated first, then it will
fail because `ruled/init.lua` doesn't exist yet. When it is done
in a separate commit, then `config.ld` is already updated and comes
with `init.lua`.

docs/config.ld

@@ -471,6 +471,7 @@ file = {
         '../lib/naughty/constants.lua',
         '../lib/naughty/dbus.lua',
         '../lib/beautiful/gtk.lua',
+        '../lib/ruled/init.lua',
 
         -- Ignore some parts of the widget library
         '../lib/awful/widget/init.lua',

lib/ruled/init.lua

@@ -0,0 +1 @@
+return {}