Added README

README.md

+pystencils-walberla
+===================
+[![pipeline status](https://i10git.cs.fau.de/pycodegen/pystencils_walberla/badges/master/pipeline.svg)](https://i10git.cs.fau.de/pycodegen/pystencils_walberla/commits/master)
+
+Formulate stencil kernels in pystencils and generate optimized C++ Code for waLBerla from it.
+Look at the waLBerla documentation on how to use it.
+
+Install with
+```bash
+pip install pystencils-walberla
+```
+and enable WALBERLA_BUILD_WITH_CODEGEN in waLBerla's CMake.
+
+