Setup instructions should mention CoreData Framework needs to be added to project

[mikecleach]

On building my project I got this error:
Undefined symbols for architecture i386:
"OBJC_CLASS$_NSAttributeDescription", referenced from:
objc-class-ref in HVCoreDataHandler.o
"OBJC_CLASS$_NSEntityDescription", referenced from:
objc-class-ref in HVCoreDataHandler.o
"OBJC_CLASS$_NSFetchRequest", referenced from:
objc-class-ref in HVCoreDataHandler.o
"OBJC_CLASS$_NSRelationshipDescription", referenced from:
objc-class-ref in HVCoreDataHandler.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)

A stack overflow post pointed out that it's because CoreData framework needs to be added. This should probably be mentioned in the installation notes.