Proposing the "delegate lookup" feature

Hi,

Following the long discussion we had about the [ParentAwareContainerInterface](https://github.com/container-interop/container-interop/pull/8), I think we all agreed it was a bad idea to have this feature exposed in an interface (because a setter does not belong to an interface.

Therefore, I have simply decided to expose the idea of this feature in a separate document. The idea is simply to incite DI containers developer to add this feature to their container (since it allows several containers to share instances in the same application).

I tried to describe the feature as simply as possible. I used a few images to make the understanding easier. I'm pretty sure the explanation could be better (I'm not an English native speaker), so I certainly need some feedback.

Please let me know what you think about this.