add default snowflake lib and shell as example

shells/default/default.nix

@@ -0,0 +1,14 @@
+{
+# Snowfall Lib provides a customized `lib` instance with access to your flake's library
+# as well as the libraries available from your flake's inputs.
+lib,
+# You also have access to your flake's inputs.
+inputs,
+
+# All other arguments come from NixPkgs. You can use `pkgs` to pull shells or helpers
+# programmatically or you may add the named attributes as arguments here.
+pkgs, stdenv, ... }:
+
+pkgs.mkShell {
+  # Create your shell
+}