Say how to build and install from source, not only how to run it

The README gave cargo run and nothing else. A binary run from target/
finds no models and, in a release build, no manual, because both are
looked up only where an install puts them. Spell out the Arch package,
a /usr/local install mirroring the PKGBUILD, the Windows installer
cross-build and where it lands, and the Android APK.

README.md

@@ -83,21 +83,118 @@ texture directly — no readback between the GPU and the screen.
 | Windows | `DarkRoom-<version>-x86_64-setup.exe`, cross-built by CI ([windows.md](docs/dev/windows.md)) | Verified under Wine only; unsigned |
 | Flatpak | [`packaging/flatpak/`](packaging/flatpak/) | Manifest in tree; folders are chosen through the portal, but no Flatpak has been built to prove it |
 
-Or build it. Git LFS is required for the model weights, and the toolchain
-pins itself to 1.92.0:
+## Building from source
+
+**Before anything.** Git LFS holds the model weights and the manual's
+pictures; a clone without it has ~130-byte pointers in their place, and every
+packager below refuses to ship one. The Rust toolchain pins itself to 1.92.0
+through `rust-toolchain.toml`, so rustup is all you install. Slint needs a few
+system headers, and the app needs a Vulkan driver at runtime:
 
 ```bash
+git clone https://gitea.tourolle.paris/dtourolle/DarkRoom.git && cd DarkRoom
 git lfs install && git lfs pull
+
+# Debian / Ubuntu
+sudo apt-get install pkg-config libfontconfig1-dev libxkbcommon-dev libvulkan1
+# Arch
+sudo pacman -S --needed pkgconf fontconfig libxkbcommon vulkan-icd-loader
+```
+
+**To try it** from the checkout, without installing anything:
+
+```bash
 cargo run --release -p darkroom-desktop
 ```
 
-Android, through the containerised toolchain ([docker/android](docker/android/README.md)):
+This is for development. The binary under `target/` finds no face, scene or
+panorama-fill models, and a release build does not find the manual either:
+it looks for all of them in the system data directories an install creates
+(`$XDG_DATA_DIRS/darkroom`, by default `/usr/local/share/darkroom` and
+`/usr/share/darkroom`), never in the checkout. Those features show as
+unavailable until it is installed.
+
+### Linux: build and install
+
+**On Arch**, build a package from the checkout and install it with pacman,
+so it can be upgraded and removed like any other:
 
 ```bash
-./docker/android/build.sh cargo ndk -t arm64-v8a build --release
+cd packaging && makepkg -si
 ```
 
-[CONTRIBUTING.md](CONTRIBUTING.md) has the system packages, the four
+**Elsewhere**, build the release binary and install it under `/usr/local`
+by hand. These are the same files, in the same places, as the Arch package
+([`packaging/PKGBUILD`](packaging/PKGBUILD)'s `package()` is the reference):
+
+```bash
+cargo build --release --locked -p darkroom-desktop
+# -> target/release/darkroom-desktop
+
+P=/usr/local
+sudo install -Dm755 target/release/darkroom-desktop $P/bin/darkroom-desktop
+
+# The models: faces and eye state, scene categories, panorama border fill
+sudo install -d $P/share/darkroom/models
+sudo install -m644 models/face/*.onnx models/scene/* models/inpaint/*.onnx \
+    $P/share/darkroom/models/
+
+# The offline manual the Help menu opens
+sudo install -Dm644 docs/manual/index.html $P/share/darkroom/manual/index.html
+sudo install -Dm644 -t $P/share/darkroom/manual/media docs/manual/media/*
+
+# Launcher entry, icon and software-centre description
+sudo install -Dm644 packaging/paris.tourolle.darkroom.desktop \
+    $P/share/applications/paris.tourolle.darkroom.desktop
+sudo install -Dm644 ui/dr-ui/ui/app-icon.png \
+    $P/share/icons/hicolor/256x256/apps/paris.tourolle.darkroom.png
+sudo install -Dm644 packaging/paris.tourolle.darkroom.metainfo.xml \
+    $P/share/metainfo/paris.tourolle.darkroom.metainfo.xml
+```
+
+Then run `darkroom-desktop`, or open it from the application menu. To
+uninstall, remove those files and `/usr/local/share/darkroom`. Your catalog,
+settings and thumbnails live in `darkroom/` under your own XDG data, config
+and cache directories (`~/.local/share`, `~/.config`, `~/.cache`) and are
+not touched by either.
+
+Optional at runtime: `gnome-keyring` or `kwallet` to remember Nextcloud
+credentials, and an ONNX Runtime in `/usr/lib` (CPU, or ROCm on an AMD GPU) to
+run the models on every core rather than on the built-in engine.
+
+### Windows: build the installer
+
+The `.exe` is cross-built from Linux in a container (podman or docker), with
+no Windows machine involved. Two steps — the executable, then the NSIS
+installer that carries it with its models and manual:
+
+```bash
+./docker/windows/build.sh cargo build --release --target x86_64-pc-windows-gnu -p darkroom-desktop
+./docker/windows/build.sh docker/windows/package.sh
+```
+
+Both land in the container's cache on the host, `~/.cache/darkroom-windows/target/`:
+the bare executable under `x86_64-pc-windows-gnu/release/darkroom-desktop.exe`,
+the installer under `installer/DarkRoom-<version>-x86_64-setup.exe`.
+Copy that to the Windows machine and run it — it installs per user, needs no
+administrator rights, and adds an uninstaller. Run on its own, the bare
+`.exe` looks for `models\` and `manual\` beside itself, so use the installer.
+[docker/windows](docker/windows/README.md) has the details.
+
+### Android: build the APK
+
+Also containerised ([docker/android](docker/android/README.md)). This
+builds, packages and debug-signs the APK, and with `--install` puts it on a
+device connected over adb:
+
+```bash
+./docker/android/package.sh --install
+```
+
+A debug-signed APK cannot replace one installed from a release; uninstall
+that first.
+
+[CONTRIBUTING.md](CONTRIBUTING.md) has the four
 commands CI runs against what you send, and the shortest useful
 contribution — a develop operation is one YAML file, and it arrives with its
 controls, its place in the chain and its tests.