Hi,
I did this in crm.lead by doing 

class CrmLead(models.Model):
    _name = 'crm.lead'
    _inherit = ['crm.lead', 'documents.mixin']

then just use the methods from document.mixin to create a new folder and set other parameters

def _get_document_folder(self):
    return self.env['documents.folder'].create({'name': 'Crm'})

def _get_document_partner(self):
    return self.partner_id