document nix and add other pkgs

Author: LucioFranco

README.md

@@ -101,6 +101,12 @@ source .env/bin/activate
 pip3 install maturin pyperf pytest
 ```
 
+Or you can use NIX to drop you into a shell with everything installed
+
+```
+nix-shell
+```
+
 Build the development version and use it:
 
 ```

shell.nix

@@ -5,6 +5,8 @@
     python39
     python39Packages.pip
     python39Packages.virtualenv
+    python39Packages.pytest
+    python39Packages.pyperf
     maturin
   ]);
   runScript = "bash";