I added a README file in order to document the process of mapping a
current legacy database with Django. It's required only at the start of
the project or creation of a new app calling a legacy database. This
helps to start the historic of the migrations to the current state of
the database.

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>

Update the of README file

I updated the documentation to precise the fact that the migration
process must be applied for each app calling the legacy database.

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>