Add missing documentation for packaging.pypi.base and .simple
diff --git a/Doc/library/packaging.pypi.rst b/Doc/library/packaging.pypi.rst
index 93b61c9..14602ce 100644
--- a/Doc/library/packaging.pypi.rst
+++ b/Doc/library/packaging.pypi.rst
@@ -51,3 +51,24 @@
.. method:: get_release
.. method:: get_releases
+
+
+:mod:`packaging.pypi.base` --- Base class for index crawlers
+============================================================
+
+.. module:: packaging.pypi.base
+ :synopsis: Base class used to implement crawlers.
+
+
+.. class:: BaseClient(prefer_final, prefer_source)
+
+ Base class containing common methods for the index crawlers or clients. One
+ method is currently defined:
+
+ .. method:: download_distribution(requirements, temp_path=None, \
+ prefer_source=None, prefer_final=None)
+
+ Download a distribution from the last release according to the
+ requirements. If *temp_path* is provided, download to this path,
+ otherwise, create a temporary directory for the download. If a release is
+ found, the full path to the downloaded file is returned.