Fixed #23530 -- Specified PyYAML requirement in initial-data.txt

Thanks aks for the report.

docs/howto/initial-data.txt

@@ -21,10 +21,12 @@ Providing initial data with fixtures
 A fixture is a collection of data that Django knows how to import into a
 database. The most straightforward way of creating a fixture if you've already
 got some data is to use the :djadmin:`manage.py dumpdata <dumpdata>` command.
-Or, you can write fixtures by hand; fixtures can be written as XML, YAML, or
-JSON documents. The :doc:`serialization documentation </topics/serialization>`
-has more details about each of these supported :ref:`serialization formats
-<serialization-formats>`.
+Or, you can write fixtures by hand; fixtures can be written as JSON, XML or YAML
+(with PyYAML_ installed) documents. The :doc:`serialization documentation
+</topics/serialization>` has more details about each of these supported
+:ref:`serialization formats <serialization-formats>`.
+
+.. _PyYAML: http://www.pyyaml.org/
 
 As an example, though, here's what a fixture for a simple ``Person`` model might
 look like in JSON: