Provide example for using patch.crates_io argument

[clauswilke]

Currently the documentation doesn't explain how to use the patch.crates_io argument in rust_function() etc. I think we should do so and also provide a simple example, e.g. with how to use the github version of the extendr api.

Key point is it is a list of lists:

patch.crates_io = list(`extendr-api` = list(git = "https://github.com/extendr/extendr"))