Added note about performance into the readme

Author: staabm

README.md

@@ -4,3 +4,8 @@ APCu Backwards Compatibility Module
 [![Build Status](https://travis-ci.org/krakjoe/apcu-bc.svg?branch=master)](https://travis-ci.org/krakjoe/apcu-bc)
 
 This module provides a backwards APC compatible API using [APCu](https://github.com/krakjoe/apcu).
+
+Performance
+===========
+
+There is no appreciable difference; This layer calls the same internal (to apc) functions that APCu does, but just accepts the old parameters, the only meaningful difference is prototypes and names of functions.