Doc: Added a \note about the JAVA_HOME variable

Without this variable set to the JDK install dir,
Qt Creator cannot find the Java binaries required
to build the application.

Task-number: QTBUG-30173
Change-Id: Id839b6ddc3c60dd09f29a8b47a77a9d147145b3a
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>