Add README for DDG4 and DDDigi

DDDigi/README.md

@@ -3,7 +3,7 @@
 DDDigi: The digitization tools of DD4hep
 ========================================
 
-`DDDigi` is supposed to interprete the results of the detector simulation phase
+DDDigi is supposed to interprete the results of the detector simulation phase
 e.g. using the simulation toolket [DDG4](../DDG4) with its command line interface
 [ddsim](../DDG4/python/DDSim).
 This is the final phase of data processing to obtain data from the simulation 
@@ -22,23 +22,32 @@ apparatus such as
 - conversion of the final signal data into the equivalent of ADC counts.
 - ...
 
+In addition these effects should be computed in the presence of
 
+- multiple interactions in the same beam-crossing
+- spillover signal from previous or following interactions due to the
+  relaxation time of the apparatus.
 
+DDDigi addresses these issues in a very modular way and implements a multi-threaded approach
+to perform such data processing.
+To support multi-threading and to avoid any complication for users due to data races 
+the following basic assumptions were made:
 
-This detector design has been pulled from , and was originally developed in
-[ATHENA](https://eicweb.phy.anl.gov/EIC/detectors/athena), for the Electron-Ion Collider.
+- The signals from disjunct subdetectors are generally independent
+  This means the detector response of any tracking subdetector is uncorrelated with
+  the detector response of e.g. a calorimeter.
+- Within a subdetector the response of disjunct units are uncorrelated. This means that 
+  e.g. hits on one layer of a layered tracking device are uncorrelated 
+  with the hits produced in another layer.
+- Within one such a layered device the response of individual sensors is tyically uncorrelated.
 
-![PFRICH](doc/geometry.png)
+These assumptions however are not strict, but rather require input from the designer of such
+a digitization application for a given appratus. These basic assumption only show the maximal
+level of parallelization possible when processing data signals.
+The parallelization can be configured for each subdetector at each level according to 
+boundary conditions e.g. given by cross-talk or the data volume arising from the
+energy deposits of the simulation.
 
-To use `ctest`, run:
-```bash
-cd ..  # `pwd` should now be `DD4hep/examples`
-mkdir build
-cd build
-cmake -DDD4HEP_EXAMPLES="OpticalTracker" .. && make && make install
-ctest --output-on-failure   # or use `--verbose` to see all output
-```
 
 
-
-![HORIZON2020](../doc/usermanuals/DDG4/setup/Horizon2020-grant-logo.png)
+![HORIZON2020](../doc/usermanuals/DD4hep/figures/AIDAinnova.png)

DDG4/README.md

 ![DDG4](../doc/logo_small.png)
 
-DDG4: The digitization tools of DD4hep
-========================================
+DDG4: The simulation toolkit of DD4hep
+======================================
 
 Simulating the detector response is an essential tool in high energy physics
 to analyze the sensitivity of an experiment to the underlying physics.
 Such simulation tools require a detailed though convenient detector description as 
-it is provided by the `DD4hep` toolkit `DDG4` which uses [Geant4](https://geant4.web.cern.ch/)
+it is provided by the DD4hep toolkit DDG4 which uses [Geant4](https://geant4.web.cern.ch/)
 to simulate the energy response of a given apparatus.
-The toolkit implements a modular and flexible approach to simulation activities using `Geant4`.
-User defined simulation applications using `DDG4` can easily be configured, 
+The toolkit implements a modular and flexible approach to simulation activities using Geant4.
+User defined simulation applications using DDG4 can easily be configured, 
 extended using specialized action routines.
 
 The design is strongly driven by easy of use;
@@ -17,9 +17,15 @@ developers of detector descriptions and applications using
 them should provide minimal information and minimal specific
 code to achieve the desired result.
 
-For further details please see the user manual 
+DDG4 also supports the fast Geant4 simulation interfaces GFlash and FastSim
+to speed up certain simulation procedures.
+
+For further details please see the user manual in the formats
 [HTML](https://dd4hep.web.cern.ch/dd4hep/usermanuals/DDG4Manual/DDG4Manual.html)
+and
 [PDF](https://dd4hep.web.cern.ch/dd4hep/usermanuals/DDG4Manual/DDG4Manual.pdf)
 
+Some usage examples for DDG4 can be found [here](../examples/DDG4)
+
 
-![HORIZON2020](../doc/usermanuals/DDG4/setup/Horizon2020-grant-logo.png)
+![HORIZON2020](../doc/usermanuals/DD4hep/figures/AIDA-2020.png)