Documentation of rcpsp minizinc models

[g-poveda]

    Nice! 

A new minizinc model is labelled "single_resource" but there is also another one called "single-resource". It may be a bit confusing... (that being said "single-resource" is never tested so maybe never used?)

It underlines the fact that these different options for model_type are not documented anywhere I think. Probably something to work on in a later PR.

Originally posted by @nhuet in #45 (comment)