[DOC] Add overview for Saros core and multiple IDEs

The new picture should illustrate what parts of the repository make up
which Saros version.
The core project is necessary for all versions (currently, there are
only two versions: Eclipse and IntelliJ IDEA).

Change-Id: I2604e262e54ddf50816664642dd361ac6111ccd1
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1465
Tested-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>