Document nix devshells in README

This only explains how to obtain different rust toolchains using direnv
or `nix develop`, since flake checks are currently limited by
redis/docker issue.

README.md

@@ -110,6 +110,18 @@ cargo update -p zstd-sys --precise 2.0.8+zstd.1.5.5
 
 ## Contributing
 
+### Nix Development Shells
+
+Where nix is available (NixOS or
+[otherwise](https://determinate.systems/nix-installer/)), development shells are provided.
+
+The default shell uses rust nightly, and can be activated manually using `nix
+develop` in the project root, or automatically with
+[direnv](https://determinate.systems/posts/nix-direnv/).
+
+To use the minimal supported version, use `nix develop .#msrv`. `.#stable` is
+also provided.
+
 ### Testing
 
 We test a few different features combinations in CI. To run all of the combinations locally, have Docker running and run `contrib/test.sh`.