Add documentation for how to stub R6 methods

[daattali]

The README simply says "This also works with R6 classes and methods." and there is no other documentation or mentions of R6 anywhere. It would be useful to show an example.

[maciekbanas]

E.g. I would like to know how to mock R6 private methods. (is it possible, or only public?)

[hadley]

mockery is no longer under active development so I'm closing this issue. We now recommend testthat::local_mocked_binding() instead, but unfortunately that doesn't help with R6 (yet).

[hadley]

I've filed an issue to look into this for testthat: r-lib/testthat#1892