Document how to update installation instructions

[maelle]

On https://r-multiverse.org/contributors.html could we clarify two items package maintainers might consider doing

• adding a README badge (it's already documented)
• adding two code chunks instructing users how to install the package from either community or production. For the latter the maintainer needs to wait for the first production snapshot after their package has been registered.

[wlandau]

I agree and proposed https://github.com/r-multiverse/r-multiverse.github.io/compare/169

[wlandau]

Fixed in r-multiverse/r-multiverse.github.io#73