migration: update docs

Update the migration docs:

Among other changes:
  * Added a general list of advice for device authors
  * Reordered the section on conditional state (subsections etc)
    into the order we prefer.
  * Add a note about firmware

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20180503191059.19576-1-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>