[DOC] added JTourbus for using custom messages

Added a detailed tour with some technical stuff on how to create custom
/ new messages to avoid the failing of former developers to fix logic in
Saros which could not be fixed without additional information gained
through extending the existing network protocol.

Change-Id: Iefcb7ed0a010e491a6a686b985ec4c41082f528c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1203
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>