Type Variance

[jeremieSimon]

I would like to add a bit more details about the variance in the category "Type and Generics".
Specifically, a rule of thumb for when to use invariance, covariance and contravariance.
I think that the explanation provided for covariance and invariance is great but this section is missing an explanation on contravariance. I would like to contribute to this section and explain how and when to use it.