Read-only field in a CI feature is not implemented yet .. what you can do if you're not going to change it during deployment is to define a hidden field with a default value.
<type-modification type="udm.DeploymentPackage">
<property name="notes" kind="string" default="My Custom Notes" hidden="true"/>
If you want this to be visible then you will need to define 2 properties one visible and another hidden.
The visible would be a display text field.
The hidden would be what is actually used.
You can then make sure the description is clear that even if a user changes this value, it will remain at the default.
<type-modification type="udm.DeploymentPackage">
<property name="hiddenNotes" kind="string" default="My Custom Notes" hidden="true"/>
<property name="notes" kind="string" default="My Custom Notes" hidden="false" description="Changing this will not affect actual value, it will remain at the default"/>