Add XCode4.6 instruction to look for headers recursively

Starting at XCode 4.6, the way to set Xcode to search for headers is not a checkbox anymore. There is a dropdown menu next to the variables. It's set by default to "non-recursive". If not changed, headers are not found. Confusing. Better specify it in the installation instruction to save readers time.
![recursively](https://f.cloud.github.com/assets/376581/267935/452c450c-8ee8-11e2-8b8a-794abe221e48.png)