Add README: what this installs, and what it needs

README.md

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+# ANT server — installer
+
+Installs the ANT server stack into `~/ant-server/<version>/` and prints the two
+commands that start and stop it. Linux and macOS.
+
+```bash
+curl -fsSL https://git.djdadi.com/bav/ant-install/releases/download/v2.37/install-antserver-2.37.sh | bash
+```
+
+Add `--start` to have it start the stack as well:
+
+```bash
+curl -fsSL https://git.djdadi.com/bav/ant-install/releases/download/v2.37/install-antserver-2.37.sh | bash -s -- --start
+```
+
+The script also runs from a file, if it reached you as an attachment or on a
+USB stick — `bash install-antserver-2.37.sh`.
+
+## Before you start
+
+**Docker must be installed and running.**
+
+- macOS — [Docker Desktop](https://www.docker.com/products/docker-desktop). It
+  must be running before you install, every time — it does not start itself
+  after a reboot.
+- Linux — [Docker Engine](https://docs.docker.com/engine/install/). Add
+  yourself to the `docker` group so it runs without `sudo`, then log out and
+  back in.
+
+**You need to be signed in to the image registry**, once per machine:
+
+```bash
+docker login git.djdadi.com
+```
+
+Ask whoever pointed you here for the username and access token. The installer
+checks this before it writes anything and tells you if it is missing, rather
+than letting it surface later as a download that never finishes.
+
+## What it does
+
+Nothing is installed system-wide, no `sudo` is used, and nothing outside
+`~/ant-server/<version>/` is written.
+
+| It does | Notes |
+|---|---|
+| picks the first free port block | `+0` if nothing holds it, otherwise `+100`, `+200`… — checked against both the other installs under `~/ant-server` and what is listening right now |
+| keeps your `.env` and `configuration/` on a re-run | the two files you might tune. Everything else is refreshed, so **re-running is how you take a fix**. `--force` replaces them too |
+| downloads about 500 MB on first start | the images, once per machine per version |
+
+Options: `--dir DIR`, `--port-offset N`, `--start`, `--force`, `--help`.
+
+## Verifying the download
+
+Each release includes a `.sha256` file:
+
+```bash
+curl -fsSLO https://git.djdadi.com/bav/ant-install/releases/download/v2.37/install-antserver-2.37.sh
+curl -fsSLO https://git.djdadi.com/bav/ant-install/releases/download/v2.37/install-antserver-2.37.sh.sha256
+sha256sum -c install-antserver-2.37.sh.sha256
+```
+
+## What is and is not public
+
+This installer is public. What it downloads is not: the compose file, the
+configuration and the database seed are pulled from `git.djdadi.com` as a
+container image, pinned by digest, behind the same sign-in as the server
+images. The ANT server software is confidential and licensed from BlueBotics —
+do not pass it on.
+
+Windows is not covered here; ask for the zip bundle, which has double-click
+entry points instead.