docs: clarify optional nix-direnv integration

Author: thebrandonlucas

CONTRIBUTING.md

@@ -9,17 +9,18 @@ for your operating system from the
 
 ## Nix Development Environment
 
-The repository flake provides Roc, the Rust toolchain and cross-compilation
+The repository flake provides Roc nightly, the Rust toolchain and cross-compilation
 standard libraries, Zig, Python, and the documentation preview server on
 supported Linux and macOS systems. Enter it with:
 
 ```sh
 nix develop
 ```
 
-Alternatively, install both [`direnv`](https://direnv.net/) and
-[`nix-direnv`](https://github.com/nix-community/nix-direnv), then approve the
-checked-in `.envrc` once:
+To load the shell automatically, install [`direnv`](https://direnv.net/) and
+hook it into your shell. The checked-in `.envrc` uses direnv's `use flake`;
+install [`nix-direnv`](https://github.com/nix-community/nix-direnv) as well if
+your direnv version does not provide it. Then approve `.envrc` once:
 
 ```sh
 direnv allow