add minimal documentation in READMe.md

README.md

@@ -8,3 +8,48 @@
 
 
 
+## Overview
+
+This simple example creates a layered sandwich calorimeter that can be used to run either a GFlash parameterisation
+or a standard Geant4 simulation for the detector response. The detector geometry is very simple and consists of
+alternating absorber and sensitive layers that can be made of differen - or the same - materials. The later can be
+used to study the effect of the `GFlashHomoShowerParameterisation`. Optionally the `GFlashSamplingShowerParameterisation` is used.
+
+The parameters for the detector geometry (incl. readout cell sizes) are defined in [./include/SimpleCaloConfig.hh](./include/SimpleCaloConfig.hh).
+Also materials and the Gflash parameterisation to be used are defined there. (Recompilation is needed for changes to take effect.
+
+
+## Building the example
+
+The example can be build with the usual `cmake` workflow, e.g:
+
+```sh
+cd SimpleCalo
+mkdir build
+cmake ..
+make -j 4 install
+```
+
+## Running the example
+
+There example can be run either with an interactive `ui`:
+
+```sh
+../bin/SimpleCalo
+```
+
+where the macro `vis.mac` is executed - or in batch mode running a macro given on the command line, e.g.
+
+```sh
+../bin/SimpleCalo test.mac
+```
+
+In `test.mac` you can select the particle type, energy and wether GFlash (`/GFlash/flag 1`) or Geant4 (`/GFlash/flag 0`) are used.
+
+
+## Output
+
+The example creates an [EDM4hep](https://github.com/key4hep/EDM4hep) ROOT file `SimpleCaloEDM4hep.root` with two collections:
+`MCParticles` with the incoming true particle and `SimCalorimeterHits` with the individual hits created during the simulation.
+
+This file can be further analysed with `ROOT` as usual. See [./draw_simhits.C](./draw_simhits.C) for an example.