Added note on selecting 32-bit architecture with Xcode.

Docs/GettingStarted.dox

@@ -22,6 +22,8 @@ Urho3D uses CMake (http://www.cmake.org) to build. The process has two steps:
 
 The build process will also compile models and shaders from the Source_Asset directory into Bin/Data/Models & Bin/Data/Shaders. After the build is complete, the programs can be run from the Bin directory.
 
+When using Xcode on Mac OS X, select Project -> %Set Active Architecture -> i386 before building. Compiling Urho3D as 64-bit is not supported.
+
 To run Urho3D from the Visual Studio debugger, set it as a startup project and enter its relative path and filename into Properties -> Debugging -> Command: ..\Bin\Urho3D.exe or ..\Bin\Urho3D_d.exe. Additionally, entering -w into Debugging -> Command Arguments is highly recommended. This enables startup in windowed mode: without it running into an exception or breakpoint will be obnoxious as the mouse cursor will most probably be hidden.
 
 To actually make Urho3D.exe do something useful, it must be supplied with the name of the script file it should load and run. You can try for example the following arguments: Scripts/TestScene.as -w

Readme.txt

@@ -108,6 +108,9 @@ directory into Bin/Data/Models & Bin/Data/Shaders. On Windows & Direct3D9,
 shader compilation requires the D3DX library from the DirectX runtime or SDK to
 be available.
 
+When using Xcode on Mac OS X, select Project -> Set Active Architecture -> i386
+before building. Compiling Urho3D as 64-bit is not supported.
+
 After the build is complete, the programs can be run from the Bin directory.
 
 To run Urho3D from the Visual Studio debugger, set it as a startup project and