Pre-draft proposal for ContainerInterface

Hello everyone,

Before proposing the `ContainerInterface` as a draft to the PHP-FIG (as suggested in #16), I thought it would be a good idea to write a "pre-draft".

Here is my first attempt at it: https://github.com/moufmouf/container-interop/blob/psr-proposal/docs/ContainerInterface.md

 I'd like to gather as many comments as possible before advertising the project on the FIG mailing list.

Do you think the document conveys the right level of information? Should it be more detailed? Would you suggest better wording?

I'm open to absolutely all comments and I need your help to achieve a draft that is good enough to become a standard! So please, do not hesitate to send your  comments, changes, etc...!