how to make a field read only for the users in odoo
<record id="view_order_form_cust_ref_readonly" model="ir.ui.view">
<field name="name">sale.order.form.readonly.cust</field>
<field name="model">sale.order</field>
<field name="inherit_id" ref="sale.view_order_form"/>
<field name="groups_id" eval="[(6, 0, [ref('base.group_financial') ])]"/>
<field name="arch" type="xml">
<field name='client_order_ref' position="attributes">
<attribute name="readonly">0</attribute>
</field>
</field>
</record>