Document co / contravariance (`<+a>`, `<-a>`)

[ryyppy]

This one is definitely an advanced topic.

• What does + and - mean?
• What syntax constructs are possible? (type, function, etc.)
• What practical use-cases does it have / Why do we need it?

Things to avoid:

• Complex / too scientific wording
• Too generic examples without practical context

Hints:

• We use a covariant type in Js.Promise.t (see it here)

[ryyppy]

Probably also related to subtyping (#150)

[tom-sherman]

I found that the tour of scala has a great introduction to variance: https://docs.scala-lang.org/tour/variances.html

[aganoza]

Flow documentation also has good examples https://flow.org/en/docs/lang/variance/