Added step where you tell cmake where Qt5 lives.

OS-X-Build.md

@@ -30,6 +30,14 @@ cd citra
 
 ### Using CMake:
 
+First of all, you have to tell CMake where your Qt5 is installed (add this line to ~/.profile if you want to make this permanent.):
+
+```
+export CMAKE_PREFIX_PATH=/path/to/Qt5/5.X/clang_64/lib/cmake:$CMAKE_PREFIX_PATH
+```
+
+In the case that you installed Qt5 with Homebrew, this path is probably something like `/usr/local/Cellar/qt5/<specific version here>/clang_64/lib/cmake`.
+
 ```
 mkdir build
 cd build