Document our patch in _vendor/README.rst

Author: pradyunsg

src/pip/_vendor/README.rst

@@ -99,7 +99,8 @@ Modifications
 * ``setuptools`` is completely stripped to only keep ``pkg_resources``
 * ``pkg_resources`` has been modified to import its dependencies from ``pip._vendor``
 * ``packaging`` has been modified to import its dependencies from ``pip._vendor``
-* ``html5lib`` has been modified to ``import six from pip._vendor``
+* ``html5lib`` has been modified to import six from ``pip._vendor`` and
+  to prefer importing from ``collections.abc`` instead of ``collections``.
 * ``CacheControl`` has been modified to import its dependencies from ``pip._vendor``
 * ``requests`` has been modified to import its other dependencies from ``pip._vendor``
   and to *not* load ``simplejson`` (all platforms) and ``pyopenssl`` (Windows).