Rewrite the description of the `--continue` option in `scc rebase`

As hinted in openmicroscopy/snoopycrimecop#32, the workflow to be followed when `scc rebase` fails because of conflicts is not very clear and poorly documented.

This PR attempts to solve the second point by describing more thoroughly the `--continue` option of `scc rebase` and the expected workflow from the user. 

/cc @bpindelski @ximenesuk