cmake: Update the README.

extras/buildsystem/cmake/README

@@ -2,8 +2,10 @@ To use CMake:
 First, as CMake isn't the default on vlc populate its CMakeLists files:
 $ cd vlc && sh ./extras/buildsystem/cmake/scripts/convert_vlc_to_cmake.sh
 
+WARNING: You are strongly encouraged to use cmake-2.5 (cvs version)
+
 Now you can use CMake as usual:
-$ cd vlc && cmake . (or ccmake . if you want a graphical view of the settings)
+$ cd vlc && mkdir build && cd build && cmake .. (or ccmake .. if you want a graphical view of the settings)
 $ make [VERBOSE=1 if you want more details]
 $ make install