Fix help text. Added nix-shell to build locally

shell.nix

@@ -0,0 +1,11 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.openssl
+    pkgs.pkg-config
+  ];
+  shellHook = ''
+    unset OPENSSL_DIR
+  '';
+}