add nix infrastructure to build the grammar

README.md

@@ -1,4 +1,15 @@
 this will be a tree-sitter implementation for `nix-shell` files.
+
+try one of the following:
+- `nix build` => build the parser
+- `nix develop` => manually/interactively generate the parser
+  - `tree-sitter generate`
+
+non-flake equivalents:
+- `nix-build`
+- `nix-shell`
+
+
 99% of this repository is code-gen'd garbage.
 to recreate it or update it, follow this guide:
 <https://tree-sitter.github.io/tree-sitter/creating-parsers>