Add a README section about the rc module

Author: SSheldon

README.md

@@ -16,6 +16,34 @@ let is_kind: BOOL = msg_send![obj, isKindOfClass:cls];
 let _: () = msg_send![obj, release];
 ```
 
+## Reference counting
+
+The utilities of the `rc` module provide ARC-like semantics for working with
+Objective-C's reference counted objects in Rust.
+A `StrongPtr` retains an object and releases the object when dropped.
+A `WeakPtr` will not retain the object, but can be upgraded to a `StrongPtr`
+and safely fails if the object has been deallocated.
+
+``` rust
+// StrongPtr will release the object when dropped
+let obj = unsafe {
+    StrongPtr::new(msg_send![class!(NSObject), new])
+};
+
+// Cloning retains the object an additional time
+let cloned = obj.clone();
+autoreleasepool(|| {
+    // Autorelease consumes the StrongPtr, but won't
+    // actually release until the end of an autoreleasepool
+    cloned.autorelease();
+});
+
+// Weak references won't retain the object
+let weak = obj.weak();
+drop(obj);
+assert!(weak.load().is_null());
+```
+
 ## Declaring classes
 
 Classes can be declared using the `ClassDecl` struct. Instance variables and