[BUILD/DOC] add architecture description

In architecture.chralx you will find the description of the logical
components of Saros, as well as the connectors between these components.
Our Sonarqube will then check whether the actual architecture of the
code complies to the description in this file.

Change-Id: I2a73ec8e8588cf6416a5ece73c2dffbfb5926bc2
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1997
Tested-by: Jenkins CI
Reviewed-by: Arsenij Solovjev <xeper000@gmail.com>