extend Readme with cmake and python info

README.md

 # waLBerla
 
-waLBerla (widely applicable Lattice Boltzmann from Erlangen) is a massively
-parallel framework for multi physics applications. Besides its original
-objective, Lattice Boltzmann solvers for hydrodynamics, it now contains
-modules for other applications like Multigrid and rigid body dynamics
-as well. Great emphasis is placed on the interoperability between the modules
-in particular the fluid-particle coupling.
-It scales from laptops to current and future supercomputers while maintaining
-near-perfect efficiency.
+waLBerla (widely applicable Lattice Boltzmann from Erlangen) is a massively parallel framework for multi physics applications. Besides its
+original objective, Lattice Boltzmann solvers for hydrodynamics, it now contains modules for other applications like Multigrid and rigid
+body dynamics as well. Great emphasis is placed on the interoperability between the modules in particular the fluid-particle coupling. It
+scales from laptops to current and future supercomputers while maintaining near-perfect efficiency.
 
 See https://www.walberla.net/ for more information and a showcase of applications.
 
 ## Documentation and Tutorials
 
 Documentation for the C++ framework is available in
-[Doxygen](http://walberla.net/doxygen/index.html), while the Python interface
-is documented in [Sphinx](http://walberla.net/sphinx/index.html).
+[Doxygen](http://walberla.net/doxygen/index.html), while the Python interface is documented
+in [Sphinx](http://walberla.net/sphinx/index.html).
 
 ## Getting started
 
 The minimum requirements are a C++17-compliant compiler (e.g. GCC or Clang)
 and the [CMake](http://www.cmake.org)
 build system. Furthermore, you need an MPI library (like
-[Open MPI](http://www.open-mpi.org)) if you want to make use of parallel
-processing capabilities. All of these dependencies are typically available in
-your operating system's package manager.
+[Open MPI](http://www.open-mpi.org)) if you want to make use of parallel processing capabilities. All of these dependencies are typically
+available in your operating system's package manager.
+
+### CMake
+
+The typical steps, assuming your are in the waLBerla source directory, are:
+
+- `mkdir build; cd build` create a build directory and change into it
+- `cmake ..` call CMake with the waLBerla source directory as a argument
+- `make` build waLBerla
+
+To specify a CMake option you need to use `-D(Option)=(Value)`. For example to set the C++ compiler one can use:
+`cmake -DCMAKE_CXX_COMILER=clang++`
+
+To list and modify the CMake options the `ccmake` tool can be used. Just call `ccmake .` in your **build** directory to see and change the
+CMake options and variables.
+
+Some important CMake variables:
+
+- `WALBERLA_BUILD_WITH_CODEGEN` Enable pystencils code generation"
+- `Python_ROOT_DIR` Define the root directory of a Python installation.
+- `MPI_HOME` Specify the base directory of the MPI installation.`
+- `WALBERLA_BUILD_WITH_PYTHON` Support for embedding Python
+- `WALBERLA_BUILD_WITH_CUDA` Enable CUDA support
+
+For a full list of CMake Option see the [CMakeLists.txt](CMakeLists.txt) file or use `ccmake as described above.`
+
+### Codegen and Python
+
+To use the `lbmpy`/`pystencils` code generation please install the packages with e.g. `pip install lbmpy` and specify the correct python
+environment when calling CMake.
+
+In previous versions of CMake one could use `PYTHON_EXECUTABLE` or `PYTHON_ROOT_DIR` (all upper case) to specify the python executable or
+the directory. This does **NOT** work anymore. Please use `Python_ROOT_DIR`.
 
 ## Get involved
 
 ### Contributing
 
 Please submit all code contributions on our
-[GitLab](https://i10git.cs.fau.de/walberla/walberla). To get an account, please
-sign and submit the [contributor license agreement](CONTRIBUTING.txt).
+[GitLab](https://i10git.cs.fau.de/walberla/walberla). To get an account, please sign and submit
+the [contributor license agreement](CONTRIBUTING.txt).
 
 ### Support
 
@@ -48,29 +75,34 @@ Many thanks go to waLBerla's [contributors](AUTHORS.txt)
 If you use waLBerla in a publication, please cite the following articles:
 
 Overview:
-  - M. Bauer et al, *waLBerla: A block-structured high-performance framework for
-    multiphysics simulations*. Computers & Mathematics with Applications, 2020.
-    https://doi.org/10.1016/j.camwa.2020.01.007.
+
+- M. Bauer et al, *waLBerla: A block-structured high-performance framework for multiphysics simulations*. Computers & Mathematics with
+  Applications, 2020.
+  https://doi.org/10.1016/j.camwa.2020.01.007.
 
 Grid Refinement:
-  - F. Schornbaum and U. Rüde, *Massively parallel algorithms for the lattice boltzmann
-    method on nonuniform grids*. SIAM Journal on Scientific Computing, 2016.
-    https://doi.org/10.1137/15M1035240
+
+- F. Schornbaum and U. Rüde, *Massively parallel algorithms for the lattice boltzmann method on nonuniform grids*. SIAM Journal on
+  Scientific Computing, 2016.
+  https://doi.org/10.1137/15M1035240
 
 LBM - Particle Coupling:
-  - C. Rettinger and U. Rüde, *A comparative study of fluid-particle coupling methods for
-    fully resolved lattice Boltzmann simulations*. Computers & Fluids, 2017.
-    https://doi.org/10.1016/j.compfluid.2017.05.033
+
+- C. Rettinger and U. Rüde, *A comparative study of fluid-particle coupling methods for fully resolved lattice Boltzmann simulations*.
+  Computers & Fluids, 2017.
+  https://doi.org/10.1016/j.compfluid.2017.05.033
 
 MESA-PD:
-  - S. Eibl and U. Rüde, *A Modular and Extensible Software Architecture for Particle Dynamics*.
-    Proceedings Of The 8Th International Conference On Discrete Element Methods.
-    https://mercurylab.co.uk/dem8/full-papers/#page-content
+
+- S. Eibl and U. Rüde, *A Modular and Extensible Software Architecture for Particle Dynamics*. Proceedings Of The 8Th International
+  Conference On Discrete Element Methods.
+  https://mercurylab.co.uk/dem8/full-papers/#page-content
 
 Carbon Nanotubes:
-  - G. Drozdov et al, *Densification of single-walled carbon nanotube films:
-    Mesoscopic distinct element method simulations and experimental validation*.
-    Journal of Applied Physics, 2020. https://doi.org/10.1063/5.0025505
+
+- G. Drozdov et al, *Densification of single-walled carbon nanotube films:
+  Mesoscopic distinct element method simulations and experimental validation*. Journal of Applied Physics,
+  2020. https://doi.org/10.1063/5.0025505
 
 ## License