Document the need for model objects to be immutable

[robertjpayne]

New issue checklist

• I have reviewed the README and documentation
• I have searched existing issues and this is not a duplicate

General information

• IGListKit version: master
• iOS version(s): not applicable
• CocoaPods/Carthage version: not applicable
• Xcode version: 8.3
• Devices/Simulators affected: not applicable
• Reproducible in the demo project? (Yes/No): not applicable
• Related issues: [Core Data] Can IGListKit be used with Core Data and NSFetchedResultsController? #407 [Core Data] adapter.performUpdates does not trigger any diffing on NSManagedObjects #460 [Core Data] update cell on model update event? #461

This caught me out a bit, but IGListKit currently requires that an object's pointer personality changes for any type of reload action to occur.

Even if diffIdentifier changes and the isEqual value returns false if the object's pointer is the same the diff algorithm ignores it.

I understand why this is difficult to achieve but I feel like it's sort of a hidden implementation detail and there is little documentation stating that object pointer's must change for reloads to occur.

[robertjpayne]

Gah I totally missed the Diff and Equality document. Sorry

[jessesquires]

No worries! Thanks @robertjpayne ! 👍