Self-clone in bootstrap.sh so curl-pipe works; document quick start

Piping bootstrap.sh into sh left $0 as a non-path, so SCRIPT_DIR pointed
at the caller's cwd and the module scripts couldn't be found (exit 127).
When no checkout is present, clone/update the repo into ~/.dotfiles and
re-exec from there. Add a Quick start section to the README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -4,6 +4,25 @@ Configures fresh VMs (RHEL-family or Debian-family) with a consistent
 environment. Run `./bootstrap.sh` (or curl-pipe it) to apply everything;
 each module also runs standalone and is safe to re-run.
 
+## Quick start
+
+On a fresh box, apply everything with one command:
+
+```sh
+curl -fsSL https://git.nic-oconnor.com/public/dotfiles/raw/branch/main/bootstrap.sh | sh
+```
+
+When piped like this (no local checkout), `bootstrap.sh` clones the repo to
+`~/.dotfiles` and runs from there, so `git` must be installed first. Re-running
+the command updates the checkout (`git pull`) and re-applies every module.
+Override the destination or source with `DOTFILES_DIR` / `DOTFILES_REPO`.
+
+Already have a clone? Just run it directly:
+
+```sh
+./bootstrap.sh
+```
+
 ## Modules
 
 - `zsh/` — installs zsh + oh-my-zsh, drops a managed `~/.zshrc`