updated contributing.md to reflect dev_env change

CONTRIBUTING.md

@@ -4,36 +4,10 @@
 
 We provide a `default.nix` file that defines the right development environment
 if you already are a Nix user and wish to contribute to the package. This
-development environment will provide bleeding edge packages, as it is uses a
-fork of `nixpkgs` that gets updated daily (R packages included). Because this
-environment is bleeding edge, no pre-built binaries are available from
-`chache.nixos.org`, so building this environment locally is quite
-time-consuming.
-
-To speed-up this process, we also provide a cache of this environment that also
-gets updated several times a day. To use this cache, you need to have
-[cachix](https://app.cachix.org/) installed and configured. So if you wish to
-use this development environment and its associated cache to very quickly have
-the right environment to contribute to the package, run the following steps:
-
-- First, install cachix: `nix-env -iA cachix -f https://cachix.org/api/v1/install`
-- Enable the cache: `cachix use rstats-on-nix`
-
-You might get an error message stating that your user cannot use caches. Simply
-follow the instructions in the warning to give the right permissions to your
-user to use caches.
-
-Then, when you run `nix-build`, binaries will be pulled from
-`rstats-on-nix.cachix.org` and `cache.nixos.org`. Building the development
-environment is nothing more than waiting for packages to download.
-
-There is also the possibility that you cannot successfully build the development
-environment: because the `nixpkgs` fork gets updated fully automatically several
-times a day without any checks, if building a package fails, there will be no
-fix released yet.
-
-If you cannot build the development environment, please open an issue, we could
-then propose a fix upstream.
+development environment will provide the required R packages to allow you to
+work on `{rix}`. This development environment is pinned to a specific `nixpkgs`
+commit, so it does not reflect the current status on CRAN. This is why it's also
+important to make sure that contributions work on `rhub`.
 
 ## {fledge}