Add CMake Xcode build notes

Building-mruby.md

@@ -21,10 +21,11 @@ toolchain they're most productive with.
 
 **Prerequisites:**
 * CMake 2.8.8, Bison, and a build toolchain on `PATH`
-* CMake configuration and command line builds must be performed _outside_ the project root
-  directory, e.g - the `build` subirectory. If you mistakenly ran `cmake` in the root
-  dir, delete the generated `CMakeFiles/` directory and `CMakeCache.txt` file, `cd` to
-  another dir (hint: the included `build` subdir), and run `cmake` again.
+* Except when generating Xcode or Visual Studio project files for IDE-based builds, CMake
+  configuration and command line builds must be performed _outside_ mruby's project root
+  directory. If you accidentally ran `cmake` in the root directory, delete the generated
+  `CMakeFiles/` subdirectory and `CMakeCache.txt` file, `cd` to another dir (hint: the included
+  `build` subdir), and re-run `cmake`.
 
 ### Native builds from the command line
 
@@ -36,4 +37,8 @@ _in process_, see this [basic cross compiling post](http://jonforums.github.com/
 
 ### Native OS X builds from Xcode IDE
 
-_in process_, do not edit source that's been copied into the build directory (**TODO** elaborate!)
+From mruby's root directory, run `cmake -G Xcode .` to generate Xcode project files. Open the
+`mruby.xcodeproj` file in Xcode and hack away as you would with any other project. **Note:** do not
+run CMake outside the root directory in another build directory because mruby's source files will
+be _copied_ to the build directory. Any edits will occur to the _copied_ source files rather than
+the real mruby source files, resulting in a very awkward, and potentially broken, workflow.