sequence is now required on journals

Add post-install script to create a sequence for all existing journals
Update README accordingly

account_move_name_sequence/models/account_journal.py

@@ -13,6 +13,7 @@ class AccountJournal(models.Model):
         "ir.sequence",
         string="Entry Sequence",
         copy=False,
+        required=True,
         check_company=True,
         domain="[('company_id', '=', company_id)]",
         help="This sequence will be used to generate the journal entry number.",