TensorFlow: Upstreaming changes to git.

Changes:
- Update to OS X install instructions.

Base CL: 107355822

tensorflow/g3doc/get_started/os_setup.md

@@ -26,6 +26,9 @@ $ pip install https://storage.googleapis.com/tensorflow/linux/gpu/tensorflow-0.5
 
 ### Mac OS X <a class="md-anchor" id="AUTOGENERATED-mac-os-x"></a>
 
+On OS X, we recommend installing [homebrew](http://brew.sh) and `brew install
+python` before proceeding, or installing TensorFlow within [virtualenv](#virtualenv_install).
+
 ```bash
 # Only CPU-version is available at the moment.
 $ pip install https://storage.googleapis.com/tensorflow/mac/tensorflow-0.5.0-py2-none-any.whl
@@ -363,12 +366,15 @@ If you encounter:
 ImportError: No module named copyreg
 ```
 
-Solution: TensorFlow depends on protobuf which require six-1.10.0. The
-installation on some machines may only have an earlier version of six that was
-installed using distutils.  Unfortunately, upgrading a distutils installed
-project via `pip` is deprecated and may fail.  If you having difficulty
-upgrading six, we recommend playing around with tensorflow using virtualenv.
+Solution: TensorFlow depends on protobuf which require six-1.10.0. Apple's
+default python environment has six-1.4.1 and may be difficult to upgrade.
+So we recommend either installing a separate of python via homebrew:
+
+```bash
+brew install python
+```
 
+or building / using TensorFlow within `virtualenv` as described above.