[zimeon]

Fixes #620

• Clarifies text in https://ocfl.io/draft/spec/#root-hierarchies about where OCFL Objects are allowed
• Adds "3.10 Object Validation" which says objects can exist and be validated outside of Storage Root and notes validity condition (partly a placeholder for addition of more complex validation notes that will come with other v2 features)
• Adds parallel "4.7 Storage Root Validation" which notes validity conditions for root and requirement for objects within to be valid (partly a placeholder for addition of more complex validation notes that will come with other v2 features)