add some doc for webassets:

- usage of the sass compiler as discussed in PR #441
- debug mode and compilers (#481)

docs/settings.rst

@@ -477,6 +477,15 @@ will produce a minified css file with the version identifier:
 
     <link href="http://{SITEURL}/theme/css/style.min.css?b3a7c807" rel="stylesheet">
 
+The filters can be combined, for example to use the `sass` compiler and minify
+the output::
+
+.. code-block:: jinja
+
+{% assets filters="sass,cssmin", output="css/style.min.css", "css/style.scss" %}
+    <link rel="stylesheet" href="{{ ASSET_URL }}">
+{% endassets %}
+
 Another example for javascript:
 
 .. code-block:: jinja
@@ -491,6 +500,12 @@ will produce a minified and gzipped js file:
 
     <script src="http://{SITEURL}/theme/js/packed.js?00703b9d"></script>
 
+Pelican's debug mode is propagated to webassets to disable asset packaging,
+and instead work with the uncompressed assets. However, this also means that
+the `less` and `sass` files are not compiled, this should be fixed in a future
+version of webassets (cf. the related `bug report
+<https://github.com/getpelican/pelican/issues/481>`_).
+
 .. _webassets: https://github.com/miracle2k/webassets
 .. _documentation: http://webassets.readthedocs.org/en/latest/builtin_filters.html