Update virtualenv based installation docs to mention pip<8.0 requirement.

This closes #194.

doc/CHANGES.rst

@@ -8,6 +8,8 @@ http://docs.zope.org/zope2/
 2.13.27 (unreleased)
 --------------------
 
+- Update virtualenv based installation docs to mention pip<8.0 requirement.
+
 - Explicitly require Manager role for ``AltDatabaseManager``.  [maurits]
 
 

doc/INSTALL-virtualenv.rst

@@ -20,6 +20,18 @@ Create a Virtual Environment
 Install the Zope2 2.13.26 Software Packages
 -------------------------------------------
 
+First make sure to have a compatible pip version. pip 8.0 and later
+are not supported.
+
+.. code-block:: sh
+
+   $ bin/pip install -U 'pip<8.0'
+   Installing collected packages: pip
+     Found existing installation: pip 9.0.1
+       Uninstalling pip-9.0.1:
+         Successfully uninstalled pip-9.0.1
+   Successfully installed pip-7.1.2
+
 .. code-block:: sh
 
    $ bin/pip install \