Brief documentation for safe_yaml=False

Provide a note about how to get custom yaml types when loading
tests, to make sure that the availability of the functionality
is visible but not overtly recommended.

docs/source/loader.rst

@@ -13,6 +13,14 @@ build and run tests with pytest_ with some limitations described below.
 .. note:: It is also possible to run gabbi tests from the command
           line. See :doc:`runner`.
 
+.. note:: By default gabbi will load YAML files using the ``safe_load``
+          function. This means only basic YAML types are allowed in the
+          file. For most use cases this is fine. If you need custom types
+          (for example, to match NaN) it is possible to set the ``safe_yaml``
+          parameter of :meth:`~gabbi.driver.build_tests` to ``False``.
+          If custom types are used, please keep in mind that this can limit
+          the portability of the YAML files to other contexts.
+
 .. warning:: If test are being run with a runner that supports
              concurrency (such as ``testrepository``) it is critical
              that the test runner is informed of how to group the