Ok I made it working like that:

1. Create email template as described above.

2. Override 'stock.picking' adding method to send the mail:

 @api.multi
    def action_delivery_slip_send(self):
        '''
        This function opens a window to compose an email, with the delivery slip template message loaded by default
        '''
        self.ensure_one()
        ir_model_data = self.env['ir.model.data']
        try:
            template_id = ir_model_data.get_object_reference('your_module', 'external_id_of_your_template')[1]
        except ValueError:
            template_id = False
        try:
            compose_form_id = ir_model_data.get_object_reference('mail', 'email_compose_message_wizard_form')[1]
        except ValueError:
            compose_form_id = False
        ctx = dict(self.env.context or {})
        ctx.update({
            'default_model': 'stock.picking',
            'default_res_id': self.ids[0],
            'default_use_template': bool(template_id),
            'default_template_id': template_id,
            'default_composition_mode': 'comment',
        })
        return {
            'name': _('Compose Email'),
            'type': 'ir.actions.act_window',
            'view_type': 'form',
            'view_mode': 'form',
            'res_model': 'mail.compose.message',
            'views': [(compose_form_id, 'form')],
            'view_id': compose_form_id,
            'target': 'new',
            'context': ctx,
        }

3. override stock.view_picking_form adding button in the header:

<record id="your_module_view_picking_form" model="ir.ui.view">
        <field name="name">your_module.stock.picking.form</field>
        <field name="model">stock.picking</field>
        <field name="inherit_id" ref="stock.view_picking_form"/>
        <field name="arch" type="xml">
            <header position="inside">
				<button name="action_delivery_slip_send" states="done" string="Send Delivery Slip" type="object" class="oe_highlight" groups="base.group_user"/> 
            </header>
        </field>
    </record> 

Now you should have a button opening a message composing wizzard with delivery slip as attachment

You can do configuration using following method.

1. Create email template and add context in action.

2. System will add action in form view action.

Hello,

For That You Have to Create Your Own Mail Template.

Thank You.