Define `ChainRulesCore.rrule` for `ReadOnlyArray`

Suggestion:

```julia
function ChainRulesCore.rrule(::Type{ReadOnlyArray}, parent)
    read_only_array = ReadOnlyArray(parent)
    ReadOnlyArray_pullback(Δ) = NoTangent(), Δ
    read_only_array, ReadOnlyArray_pullback
end
```