Added some docs about multiple preparers.

docs/basics.rst

@@ -105,7 +105,7 @@ deserialization but before validation; it prepares a deserialized
 value for validation. Examples would be to prepend schemes that may be
 missing on url values or to filter html provided by a rich text
 editor. A preparer is not called during serialization, only during
-deserialization.
+deserialization. You can also pass a Schema Node a list of preparers.
 
 The *validator* of a schema node is called after deserialization and
 preparation ; it makes sure the value matches a constraint.  An example of
@@ -443,6 +443,23 @@ __ http://pypi.python.org/pypi/htmllaundry/
                                      preparer=htmllaundry.sanitize,
                                      validator=colander.Length(1))
 
+You can even specify multiple preparers to be run in order by passing
+a list of functions to the 'preparer' kwarg, like so:
+
+.. code-block:: python
+   :linenos:
+
+   import colander
+   # removes whitespace, newlines, and tabs from the beginning/end of a string
+   strip_whitespace = lambda v: v.strip(' \t\n\r') if v is not None else v
+   # replaces multiple spaces with a single space
+   remove_multiple_spaces = lambda v: re.sub(' +', ' ', v)
+
+   class Page(colander.MappingSchema):
+       title = colander.SchemaNode(colander.String())
+       content = colander.SchemaNode(colander.String(),
+                                     preparer=[strip_whitespace, remove_multiple_spaces],
+                                     validator=colander.Length(1))
 
 Serialization
 -------------