add dev instructions

README.md

@@ -11,6 +11,14 @@ make
 
 there's also a flake with a shell and package - if you know nix you know what to do with those
 
+## dev
+
+`nix develop` if you're cool and use nix
+
+otherwise check `nix/shell.nix` for an up-to-date list of dependencies
+
+if you want to debug the macros (good luck keeping your sanity) run `make pp` to preprocess and format the files (output in preprocessed/)
+
 ## TODO
 
 - parser generator: