Add a pointer to the DIB pypi element's README

The README is very generic; every time I follow it, I end up with a repo
that doesn't work. The pip element's readme is much more useful for
setting up a mirror that works with DIB and TripleO.

Rather than replicate content from the element here, I've just added a
pointer to the element's README.

Change-Id: Id6d0f8a75d3fc91a72c4dbb9b9628d4c80dbfb9d