Edit: This ... forum deleted my answer, and I'm not typing it again, check here please:

https://www.odoo.com/forum/help-1/add-field-in-kanban-view-account-invoice-line-165366

After creating the field in the model you need to create the view. 

Usually looks like this:

 your_model.kanban
your_model

Open any lead/opportunity

1. Activate debug mode  → Edit View: Form

2. In the form editor:

   • Drag the field ReadyOffplan into the form

3. Save

This makes it visible on the form view (both Lead & Opportunity).

 Kanban view :

1. Open CRM → Opportunities

2. Activate developer mode → Edit View: Kanban

3. Add the field inside the kanban template, for example:

4. < field name = "x_ReadyOffplan" />

5. Save