contributing: Add R- architecture-specific label prefix
contributing: Add R- architecture-specific label prefix

carllerche/nix-rust,nix-rust/nix,posborne/rust-nix,xd009642/nix,berkowski/nix,kamalmarhubi/nix-rust,berkowski/nix,MarkusJais/nix-rust,xd009642/nix,nikklassen/nix-rust,Susurrus/nix,nikklassen/nix-rust,MarkusJais/nix-rust,nikklassen/nix-rust,Susurrus/nix,carllerche/nix-rust,berkowski/nix,xd009642/nix,carllerche/nix-rust,posborne/rust-nix,geofft/nix-rust,kamalmarhubi/nix-rust,MarkusJais/nix-rust,Susurrus/nix,kamalmarhubi/nix-rust,posborne/rust-nix,asomers/nix,geofft/nix-rust,asomers/nix,nix-rust/nix,geofft/nix-rust