Made the exclusion of instance variables more explicit

Fixes kodecocodes#5

Author: ColinEberhardt

README.md

@@ -31,6 +31,7 @@ Here are some of the documents from Apple that informed the style guide. If some
  * [Underscores](#underscores)
 * [Methods](#methods)
 * [Variables](#variables)
+ * [Instance Variables](#instance-variables)
 * [Property Attributes](#property-attributes)
 * [Dot-Notation Syntax](#dot-notation-syntax)
 * [Literals](#literals)
@@ -256,7 +257,9 @@ Variables should be named as descriptively as possible. Single letter variable n
 
 Asterisks indicating pointers belong with the variable, e.g., `NSString *text` not `NSString* text` or `NSString * text`, except in the case of constants.
 
-Property definitions should be used in place of naked instance variables whenever possible. Direct instance variable access should be avoided except in initializer methods (`init`, `initWithCoder:`, etc…), `dealloc` methods and within custom setters and getters. For more information on using Accessor Methods in Initializer Methods and dealloc, see [here](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmPractical.html#//apple_ref/doc/uid/TP40004447-SW6).
+### Instance Variables
+
+[Private properties](#private-properties) should be used in place of instance variables whenever possible. Direct access to instance variables that 'back' properties should be avoided except in initializer methods (`init`, `initWithCoder:`, etc…), `dealloc` methods and within custom setters and getters. For more information on using Accessor Methods in Initializer Methods and dealloc, see [here](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmPractical.html#//apple_ref/doc/uid/TP40004447-SW6).
 
 **Preferred:**