Document how to use custom ModelForm

[yakky]

In https://github.com/ella/django-appdata/blob/master/app_data/forms.py#L248 a model_form is created from standard modelform_factory, but sometimes you'd want to provide a custom ModelForm.
Adding a model_form argument and generating the model form only if empty, would allow this.
I can provide a PR for this

[honzakral]

A PR would be welcome.

I originally envisioned people would just use the **kwargs part that we pass to the factory that can contain a form which will then be used as a superclass for the generated ModelForm (inside modelform_factory) but that is probably too complicated in some cases.

[yakky]

Have I really been so dumb?
Obviously the form kwargs works nicely!
I reworded the title, as probably it'd be useful to document how to do this (even if it's standard Django stuff)