[RFC] Overriding guide for Sylius main

[herodrigues]

As I have discussed with @pjedrzejewski in this PR #340, there is no current docs for overriding models in Sylius main.

Let's discuss how we can do that here :)

[pjedrzejewski]

I think stuff like that should go to the cookbook! So a cookbook article "How to add a field to model in Sylius" should be created. The Bundles General Guide should stay as it is and cover standalone usage.

Of course overriding them is very similar, but I think we should have these separate, to avoid confusion like in #340 and also there will always be slight differences in how main Sylius works and how individual bundles. Product catalog in main Sylius is much more complex than in SyliusProductBundle itself.

Let's keep this discussion open and hear more thoughts, but I think @herodrigues you can work on the article already if you have some time. That would be great! Just let me know. :) Thanks!