Stop being a purist

It's good enough without `--pure`. For example, I can now use my editor
within the `nix-shell`.

Author: dmarcoux

README.md

@@ -29,7 +29,7 @@ package manager.
 
 1. Copy [shell.nix](./shell.nix) at the root of your Elixir/Phoenix project.
 2. Add the content of [.gitignore](./.gitignore) to your own _.gitignore_.
-3. Start with `nix-shell --pure`.
+3. Start with `nix-shell`.
 
 ## Mise