Hi,

to notify by your custom object:

1. Make you model inherited from mail.thread. Look for example in sale.order model

2. Add message_post method in the required method (e.g. in create, or in the button method 'submit').

The example from delivery/modles/stock_picking.py:

    @api.multi
    def send_to_shipper(self):
        self.ensure_one()
        res = self.carrier_id.send_shipping(self)[0]
        self.carrier_price = res['exact_price']
        self.carrier_tracking_ref = res['tracking_number']
        order_currency = self.sale_id.currency_id or self.company_id.currency_id
        msg = _("Shipment sent to carrier %s for expedition with tracking number %s<br/>Cost: %.2f %s") % (self.carrier_id.name, self.carrier_tracking_ref, self.carrier_price, order_currency.name)
        self.message_post(body=msg)