Add install.sh: install or upgrade Megalodon as a systemd service

Downloads the release for this machine and verifies its checksum (or builds
from source), creates the service user and folders, writes and opens the
environment file, installs the hardened unit and prints the first-run admin
password. Re-running upgrades; --uninstall [--purge] removes it.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -59,7 +59,22 @@ password you set there is the RCON password.
 admin password. Keep it on your LAN or a VPN; only Megalodon needs to reach it. If Megalodon and
 the server are on the same machine or Docker network, nothing needs to be opened at all.
 
-## Running it
+## Installing on Linux (systemd)
+
+```sh
+curl -fsSL https://git.veillax.com/Veillax/megalodon/raw/branch/main/deploy/install.sh | sudo sh
+```
+
+This downloads the latest release (checking its SHA-256), installs `/usr/local/bin/megalodon`,
+creates a `megalodon` user, `/etc/megalodon/` (settings) and `/var/lib/megalodon/` (data), opens
+`/etc/megalodon/megalodon.env` in your editor, then starts the service and prints the first-run
+admin password. Run it again to upgrade; your settings are kept.
+
+Options: `--version v0.1.0`, `--listen 0.0.0.0:8480`, `--source` (build from the repository with Go
+instead), `--no-edit`, `--no-start`, and `--uninstall` (add `--purge` to delete settings and data).
+Download the script and read it first if you prefer: it is [`deploy/install.sh`](deploy/install.sh).
+
+## Running it by hand
 
 ```sh
 # download a release, or: go build ./cmd/megalodon