Removed an extra paragraph in the ContainerInterface standard

Minor change.

In the introduction, I removed:

> Frameworks and CMSs that have custom needs MAY extend the interface for their own purpose, but SHOULD remain compatible with this document.

I think this is unnecessary as implementors implement this standard or not, it's up to them (hence we don't say they should be compatible). And we don't care if they extend this interface as long as they implement it, and "allowing" them to do so feels weird.