README: add note about nixos vm

README.md

@@ -31,6 +31,16 @@ Preferrably, this UID/GID should not be root, in order to minimize the potential
 
 ## Development and testing
 
+### Nix
+
+If you have nix installed, you can test your changes in a NixOS vm by running:
+
+```bash
+nix run .#vm
+```
+
+### General setup
+
 Ensure you have a [rust toolchain](https://www.rust-lang.org/tools/install) installed.
 
 In order to set up a test instance of mariadb in a docker container, run the following command:
@@ -92,4 +102,4 @@ cargo build
   where the old program would just work by itself.
 - The order in which some things are validated (e.g. whether you own a user, whether the
   contains illegal characters, whether the user does or does not exist) might be different
-  from the original program, leading to the same command giving the errors in a different order.
+  from the original program, leading to the same command giving the errors in a different order.