cssmin compressor including doc; and a correction to the sample custom compressor

docs/compressors.rst

@@ -142,6 +142,18 @@ To us it for your stylesheets add this to your ``PIPELINE_CSS_COMPRESSOR`` ::
 
   Default to ``'--template=highest'``
 
+cssmin compressor
+=================
+
+The cssmin compressor uses the `cssmin <http://pypi.python.org/pypi/cssmin/>`_
+Python library to compress stylesheets. To use it, specify this
+``PIPELINE_CSS_COMPRESSOR`` ::
+
+  PIPELINE_CSS_COMPRESSOR = 'pipeline.compressors.cssmin.CssminCompressor'
+
+Install the cssmin library with your favorite Python package manager. E.g. ::
+
+  pip install cssmin
 
 Write your own compressor class
 ===============================

pipeline/compressors/cssmin/__init__.py

+from pipeline.compressors import CompressorBase
+
+
+class CssminCompressor(CompressorBase):
+    """
+    CSS compressor based on the Python library cssmin
+    (http://pypi.python.org/pypi/cssmin/).
+
+    """
+
+    def compress_css(self, css):
+        from cssmin import cssmin
+        return cssmin(css)