README says that make install needs nix (closes #200)

`make install` runs `make build-bin`, which runs bun through
`nix-shell -p bun`, so on a machine without nix it stops with
`nix-shell: not found`, and the README named no such prerequisite.
Getting Started now says what its commands need and that `make install`
also needs nix, with a link to its download page. "Running the backup
from cron" says the same where it introduces `make install`, and that a
bun already installed does not help. The Makefile is unchanged, as the
plan on the issue decided. TODO.md's Next Step names the four bugs that
are open instead of saying none is.

Judgement call: "node 22 or later" is the `engines` entry in package.json.

Model: opus-5-5

README.md

@@ -19,6 +19,10 @@ encrypting and uploading them back to the server.
 
 ## Getting Started
 
+The commands below need git, node 22 or later, and yarn. `make install`, which
+builds the single binary that "Running the backup from cron" uses, also needs
+[nix](https://nixos.org/download).
+
 ```bash
 git clone https://git.eeqj.de/sneak/quak.git
 cd quak
@@ -86,9 +90,12 @@ enumeration/download calls) is exported too and documented under Design below.
 ## Running the backup from cron
 
 `quak backup` never prompts, so cron can run it. `make install` builds quak as a
-single binary and copies it to `~/bin/quak`. Log in once with it, as the user
-the cron job will run as; the session is saved in that user's data directory
-(see "Session handling"):
+single binary and copies it to `~/bin/quak`. Besides what Getting Started needs,
+it needs [nix](https://nixos.org/download), even where bun is already installed:
+`make install` runs `make build-bin`, which bundles the built `dist/` with bun
+run through `nix-shell -p bun`. Log in once with `~/bin/quak`, as the user the
+cron job will run as; the session is saved in that user's data directory (see
+"Session handling"):
 
 ```bash
 ~/bin/quak login