Explain better on rebuilding images
Explain better on rebuilding images

Add a necessary stop (python setup.py install) before
rebuilding images in README.md

midonet/midonet-sandbox,xavileon/midonet-sandbox,xavileon/midonet-sandbox,xavileon/midonet-sandbox,midonet/midonet-sandbox,midonet/midonet-sandbox