Expand upon the vendor README

pip/vendor/README

-html5lib has been modified to import six from pip.vendor

pip/vendor/README.rst

+Vendored libraries should not be modified except as required to actually
+successfully vendor them.
+
+
+Modifications
+=============
+
+* html5lib has been modified to import six from pip.vendor
+
+
+Note to Downstream Distributors
+===============================
+
+Libraries are vendored/bundled inside of this directory in order to prevent
+end users from needing to manually install packages if they accidently remove
+something that pip depends on. Realizing that downstream prefers to unbundle
+packages and that with the downstream package manager a user can use that to
+fix their installation we've attempted to make it easier to unbundle the
+included distributions.
+
+All bundled packages exist in the ``pip.vendor`` namespace, and the versions
+(fetched from PyPI) that we use are located in vendor.txt. After removing
+``pip.vendor.*`` you'll also need to update the import statements that import
+these packages.