Add pre-commit hook and make format target

I've added the new local.mk to the package sources. While this
should not be needed for the build, it is the simplest solution,
and won't cause many extra rebuilds, because the file won't change
very often.

doc/manual/src/contributing/hacking.md

@@ -273,6 +273,29 @@ Configure your editor to use the `clangd` from the `.#native-clangStdenvPackages
 > Some other editors (e.g. Emacs, Vim) need a plugin to support LSP servers in general (e.g. [lsp-mode](https://github.com/emacs-lsp/lsp-mode) for Emacs and [vim-lsp](https://github.com/prabirshrestha/vim-lsp) for vim).
 > Editor-specific setup is typically opinionated, so we will not cover it here in more detail.
 
+## Formatting and pre-commit
+
+You may run the formatters as a one-off using:
+
+```console
+make format
+```
+
+If you'd like to run the formatters before every commit, install the hooks:
+
+```
+pre-commit-hooks-install
+```
+
+This installs [pre-commit](https://pre-commit.com) using [cachix/git-hooks.nix](https://github.com/cachix/git-hooks.nix).
+
+When making a commit, pay attention to the console output.
+If it fails, run `git add --patch` to approve the suggestions _and commit again_.
+
+To refresh the config, do the following:
+- if you use `make format`: stop and start your `nix develop` shell.
+- if you use the pre-commit hook: stop and start, and run `pre-commit-hooks-install` again.
+
 ## Add a release note
 
 `doc/manual/rl-next` contains release notes entries for all unreleased changes.