Create Doxygen style guide

[nwolek]

https://github.com/jamoma/JamomaDoc/tree/master/Doxygen

We need some general guidance in the above folder about how to structure comments in the C++ code so that they result in useful online documentation for the API. This would be analogous to the verbal directions that Trond and Nathan gave to Théo during the Albi workshop. Having it written and part of the repository would help encourage more people to follow it.