Hi Adrian

You can call the  default function  message post function in the create function of your model, If you need it in already existing models you can inherit the model and add the message post functionality, also you can use tracking=True for fields that track the changes for the fields in chatter 

Check this code for reference

class ModelName(models.Model):
    _name='custom.model'

    @api.model_create_multi
    def create(self, vals_list):
        res = super(ModelName, self).create(vals_list)
        res.message_post(body=_("record created"))
        return res

For more references of message post 

Odoo Message Post

Post a message to chatter

Regards