Add an article about partial type signatures in Elm

[n1k0]

nextId : List { o | id : Int } -> Int
nextId elements =
    -- Finds the next available id from a list of identified items
    elements
        |> List.map .id
        |> List.maximum
        |> Maybe.withDefault 0
        |> (+) 1

Or even:

type alias Identifiable a =
    { a | id : Int }

nextId : List (Identifiable a) -> Int
nextId elements =
    -- Finds the next available id from a list of identified items
    elements
        |> List.map .id
        |> List.maximum
        |> Maybe.withDefault 0
        |> (+) 1