document ways to get the version of `clang-format` we need on various systems

i switched from running debian sid on my old dev machine to debian testing on my new one and `clang-format-14` is no longer available in the repos for me

i found https://github.com/muttleyxd/clang-tools-static-binaries and it seems to do the trick for me, i figure it might be helpful for people on other distros and macs too so documenting that somewhere could be good (feels like the type of thing that should go in a `CONTRIBUTING.md` file but we don't have one of those)