Use `bash` instead of `sh` to run shell scripts.
We would like to support Nix users who do not typically have any shell
other than `sh` at a known path, therefore use `/usr/bin/env bash`.
Add a `githooks` directory. Copy the sample pre-commit hook into it.
Add a section to the README explaining how to take advantage of the
githooks.
The sample pre-commit hook includes checks for trailing whitespace that
we are seeing occasionally in PRs.
Done in preparation for adding a clippy githook.
Tobin C. Harding
Vojtěch Toman