Add the Mac OS X build instructions.

Building-for-Mac-OS-X.md

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+# Mac OSX Build
+
+## Minimum OS X and Xcode versions
+---
+
+Mac OS X Mountain Lion (10.8) or newer is required to build it. Mavericks is recommended.
+
+You will need to install "Command Line Tools for Xcode". Installing Xcode itself is optional if you have installed the Command Line Tools.
+
+
+## Checkout and compile Dolphin
+---
+
+To checkout Dolphin's source: 
+```bash
+git clone https://code.google.com/p/dolphin-emu ~/dolphin-emu
+cd ~/dolphin-emu
+```
+
+To build with CMake (optionally verbose): Download and install [CMake](http://www.cmake.org/cmake/resources/software.html) if you don't have it.
+
+#### CMake Notes
+
+The `-j` option can be passed to make in order to compile multiple objects at once.
+A good rule of thumb is number of CPU cores plus one. For example, on a quad core CPU `make -j5` would be a good choice.
+
+You can execute `cmake -L` to view the options that Dolphin's CMake environment supports, as well as their current and possible settings.
+
+If you have any problems compiling, use the verbose option (shown above) to give more detail. If you report a problem, at a minimum include the last screen-full of lines.
+
+
+## Optional: Ninja
+---
+
+[Ninja](http://martine.github.io/ninja/) is a replacement for Make which is a bit faster for a Dolphin-sized project and is worth considering if you intend to rebuild frequently. 
+
+After installing it, pass `-G Ninja` to cmake and use 'ninja' instead of 'make' (note that ninja is `-j` by default). 
+
+By default, clang won't show color diagnostics when not invoked from a TTY, and ninja buffers compiler output through a pipe to avoid interleaving issues.
+To fix this, pass to cmake:
+```
+-DCMAKE_CXX_FLAGS="-Xclang -fcolor-diagnostics"
+```
+
+## Keeping up to date
+---
+
+All you need to do to update to the latest Dolphin revision is the following:
+
+```bash
+cd ~/dolphin-emu/build
+git pull
+make && open Binaries/*
+```