(update) Install watchman w/ --HEAD to avoid error

Updated the docs for installing watchman with --HEAD per @wez suggestion in this issue thread facebook#239

docs/GettingStarted.md

@@ -13,7 +13,7 @@ next: tutorial
 2. New to Xcode?  [Download it](https://developer.apple.com/xcode/downloads/) from the Mac App Store.
 3. [Homebrew](http://brew.sh/) is the recommended way to install node, watchman, and flow.
 4. `brew install node`. New to [node](https://nodejs.org/) or [npm](https://docs.npmjs.com/)?
-5. `brew install watchman`. We recommend installing [watchman](https://facebook.github.io/watchman/docs/install.html), otherwise you might hit a node file watching bug.
+5. `brew install --HEAD watchman`. We recommend installing [watchman](https://facebook.github.io/watchman/docs/install.html), otherwise you might hit a node file watching bug.
 6. `brew install flow`. If you want to use [flow](http://www.flowtype.org).
 
 ## Quick start