Improve README

Currently, README seems quite clear ... for people knowing the nuts and bolts of git.
But for not-so-good developers (like me), things seems a little ... opaque.

Can you add an example of how subtrac transforms a repository (typically, I would like to see how the submodules are transformed into valid commits)? I would also like to see how to add new submodules to a project where subtrac is already used.