add readme for iOS xcode project

STK for iOS/README.md

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+##Usage
+
+1. Drag and drop STK.xcodeproj into your xcode project
+
+1. Open the project's settings, open the *Build Phases* tab. In the *Link Binary with Libraries section*, add add **libSTK.a**. 
+
+1. Make the STK static library compile the STK source files you require. Click on **STK.xcodeproj**, open the *Build Phases* tab, and add them in the *Compile Sources* section. For example, if you want to use `SineWave`, add **SineWave.cpp**. If you want to use Brass, add 
+
+1. Add the STK's **include** folder to your project's header search paths: `../stk/include/`. Use `..` for each step above you need to go. 
+
+1. Import the STK classes in the files you require:
+#import "SineWave.h"
+
+1. Rename any Objective-C files that import STK files with the **.mm** extension. E.g. **ViewController.m** —> **ViewController.mm**