Added note on how to build release in windows to doc

git-svn-id: http://www.openflipper.org/svnrepo/OpenFlipper/branches/Free@8633 383ad7c9-94d9-4d36-a494-682f7c89f535

Doxygen/Developer/building.docu

@@ -69,6 +69,9 @@
  * <li> Now you can build OpenFlipper from within Visual Studio using the newly created project file by starting Build Solution. </li>
  * </ul> 
  *
+ * Note: If you want to build OpenFlipper as release, you have to set the CMAKE_BUILD_TYPE variable to "Release" before configuring
+ * and generating the Visual studio solution file. Choose then build type "Release" in Visual Studio and build the solution.
+ *
  * \subsection cmake_bmacos Building OpenFlipper under MacOS X
  *
   * <ul>