Add more guide for mac users.

INSTALL.md

@@ -26,9 +26,11 @@ libraries. You are on your own.
     * Install zlib. Try: `sudo apt-get install zlib1g-dev`.
     * Install bzip2: `sudo apt-get install libbz2-dev`.
 * **OS X**:
-    * We encourage you to install latest C++ compiler via [homebrew](http://brew.sh/).
-        * If you're first time developer in MacOS, please run: `xcode-select --install` in your command line.
-        * run `brew tap homebrew/dupes; brew install gcc47 --use-llvm` to install gcc 4.7 (or higher).
+    * Install latest C++ compiler that supports C++ 11:
+        # Update XCode:  run `xcode-select --install` (or install it from XCode App's settting).
+        # Install via [homebrew](http://brew.sh/).
+            * If you're first time developer in MacOS, you still need to run: `xcode-select --install` in your command line.
+            * run `brew tap homebrew/dupes; brew install gcc47 --use-llvm` to install gcc 4.7 (or higher).
     * Install zlib, bzip2 and snappy libraries for compression.
     * Install gflags. We have included a script
     `build_tools/mac-install-gflags.sh`, which should automatically install it.