Add description file for how to define new materials

HowTo-DefineMaterial.txt

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+1. Define new material, prestrain and indicator function @ /dune/microstructure/materialDefinitions.hh
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+2. Add Case (Materialname) to "setupMaterial" method @ /dune/prestrainedMaterial.hh
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+3. set new materialname parameter @ /inputs/cellsolver.parset