Write the tutorial on how to autogenerate C++ code with Simulink Coder

[diegoferigo]

In https://github.com/robotology/blockfactory/commit/742273b30f15142250e4acd517abe8c244bf42df I uploaded an initial version of the files that will be used in this tutorial. We should fill the markdown file with a more detailed explanation, pointing out how to configure Simulink for autogenerating code that supports blockfactory.