Feature Request - Add "readOnly" option to fields

[gavinworks]

Currently there is no way to set fields to "readOnly: true" via Builder. Each time the field is updated via Builder the "readOnly" flag is removed from the field definition in the fields.yaml file. Small annoyance which would be nice to have fixed :)

[LukeTowers]

@gavinworks feel free to make a PR adding it to the default field properties. See https://github.com/rainlab/builder-plugin/blob/master/classes/ControlLibrary.php#L159

[bennothommo]

This is now available as of 95f3451.

[gavinworks]

We love @bennothommo !!