Add note about --externals including missing modules.

Author: thebjorn

README.rst

@@ -20,6 +20,8 @@ pydeps
 Python module dependency visualization. This package installs the ``pydeps``
 command, and normal usage will be to use it from the command line. 
 
+New in version 1.3.4: ``--externals`` will now include modules that haven't been installed (what ``modulefinder`` calls ``badmodules``).
+
 New in version 1.2.8: A shortcut for finding the direct external dependencies of
 a package was added::