Add dev documentation about PrestaShopCollection/ObjectModel

[mcdado]

As third party module developers, it is useful to be given context for learning how the inner workings of the core function.

As a developer, I tend to look inside classes for method that I might need. When I can't find how to do something, I look online, but with PrestaShop there just isn't much there.

There are two key examples: as mentioned in #24 there are important things to know about ObjectModel, but without a comprehensive and time consuming learning of the whole system, it's not easy to reason about. A contextual explanation would go a long way to educate developers.

Another one that is very important is PrestaShopCollection, which is practically undocumented. You have to look at the code (core and native modules) to know how to use it. It can solve many issues, for example when I had to develop a module that augments Cart Rules, I found that there is no direct method to get all the Cart Rules, so I had to resort to hand-writing queries, that IMHO is an anti-pattern for such a basic thing like querying Core objects. Also, other classes have "get" methods that return multidimensional arrays straight from database rows, others return objects. From what I could see, it would be PrestaShopCollection that adds the necessary consistency, à la jQuery objects… am I correct?

I think documentation should explain these areas better.