Add qt4 install instructions for osx 10.11

Author: cbrightsci

README.md

@@ -37,6 +37,7 @@ For documentation, please see: http://sciinstitute.github.io/SCIRun/
   - Apple clang 5.1
   - Qt 4.8
     + Download from http://releases.qt-project.org/qt4/source/qt-mac-opensource-4.8.4.dmg.
+    + The above dmg will not work if using OS X 10.11. In this case Qt 4.8 can be acquired through macports using the command 'sudo port install qt4-mac' - more on macports here: https://www.macports.org/install.php
 * Windows (tested on Windows 7, 8)
   - Visual Studio 2013
   - Qt 4.8