Fix NixOS instructions

Author: SuperSandro2000

README.md

@@ -165,7 +165,7 @@ bundle config build.sqlite3 --with-sqlite3-include="$(nix-store -r "$(nix-instan
 or with nix-command enabled:
 
 ```bash
-nix shell -f shell.nix
+nix develop -f shell.nix
 bundle config build.sqlite3 --with-sqlite3-include="$(nix eval nixpkgs#sqlite.dev)/include" --with-sqlite3-lib="$(nix eval nixpkgs#sqlite.out)/lib"
 ```
 

shell.nix

@@ -2,12 +2,13 @@
 
 with pkgs;
 
-mkShellNoCC {
+mkShell {
   buildInputs = [
     bintools
     gcc
     libffi.dev
     nodejs
+    postgresql
     ruby
     yarn
   ];