docs: document packaging and system-wide installation

Describe installing from the Arch and Debian packages, system-wide installs via PREFIX/DESTDIR, the packaging release flow, and add the new directories to the project layout.

README.md

@@ -41,24 +41,67 @@ already seen so nothing is delivered twice.
 
 ## Installation
 
+### Arch Linux
+
+A `PKGBUILD` lives in [`packaging/archlinux`](packaging/archlinux). Build and
+install the package with `makepkg`:
+
+```sh
+git clone https://src.panahifar.ir/ahp/lazyntfy.git
+cd lazyntfy/packaging/archlinux
+makepkg -si
+```
+
+This installs `/usr/bin/lazyntfy`, the man page under `/usr/share/man`, the
+license, and the README, and pulls in `ca-certificates` so TLS to your ntfy
+server works.
+
+### Debian and Ubuntu
+
+A debhelper-free source package lives in [`debian`](debian). Build a `.deb` and
+install it system-wide:
+
+```sh
+git clone https://src.panahifar.ir/ahp/lazyntfy.git
+cd lazyntfy
+make deb                       # writes dist/lazyntfy_<version>_<arch>.deb
+sudo apt install ./dist/lazyntfy_*.deb
+```
+
+`apt install ./file.deb` (or `dpkg -i` followed by `apt -f install`) resolves
+the `ca-certificates` dependency. The packaging needs only the Go toolchain and
+`dpkg-dev`; `make deb` passes `-d` so it also builds where `golang-go` is not
+installed as a distribution package.
+
 ### From source
 
 ```sh
 git clone https://src.panahifar.ir/ahp/lazyntfy.git
 cd lazyntfy
-make install            # installs the binary and the man page under ~/.local
+make install                   # into ~/.local, no root needed
 ```
 
-`make install` honors `PREFIX` (default `~/.local`):
+`make install` honors `PREFIX`, `BINDIR`, `MANDIR`, `DOCDIR`, and `DESTDIR`.
+Picking a system prefix installs for every user:
 
 ```sh
-make install PREFIX=/usr/local
+sudo make install PREFIX=/usr/local   # /usr/local/bin/lazyntfy, /usr/local/share/man/man1
+sudo make install PREFIX=/usr         # /usr/bin/lazyntfy, /usr/share/man/man1
+```
+
+Stage a filesystem tree into a temporary root instead of the live system (handy
+for building your own package):
+
+```sh
+make install DESTDIR="$PWD/stage" PREFIX=/usr
 ```
 
 ### With `go install`
 
+From a checkout:
+
 ```sh
-go install ./cmd/lazyntfy@latest   # or: go install lazyntfy/cmd/lazyntfy
+go install ./cmd/lazyntfy      # binary in $(go env GOPATH)/bin
 ```
 
 ## Usage
@@ -131,6 +174,25 @@ messages are pruned on startup. Credentials are never written to the database
 the current run's authentication is always used. History is best effort: if the
 file cannot be opened, the client still runs.
 
+## Packaging
+
+Both package formats are defined in the repository. `make deb` builds a Debian
+package from the working tree; the Arch `PKGBUILD` builds the `v$pkgver` release
+archive from the Gitea instance.
+
+| Format | Build | Definition |
+| --- | --- | --- |
+| Debian / Ubuntu | `make deb` → `dist/*.deb` | [`debian/`](debian) |
+| Arch Linux | `make arch` → `makepkg` | [`packaging/archlinux/PKGBUILD`](packaging/archlinux/PKGBUILD) |
+
+When cutting a release:
+
+1. Bump the version in [`debian/changelog`](debian/changelog) and `pkgver` in
+   the `PKGBUILD`.
+2. Tag the commit (`git tag -s vX.Y.Z`) and push it.
+3. In `packaging/archlinux`, run `updpkgsums` to refresh the tarball checksum and
+   `makepkg --printsrcinfo > .SRCINFO`.
+
 ## Development
 
 ```sh
@@ -162,6 +224,8 @@ go test ./internal/history/...
 | `internal/textutil` | width-aware text wrapping and clipping |
 | `internal/tui` | Bubble Tea interface |
 | `man` | `lazyntfy(1)` man page |
+| `debian` | Debian / Ubuntu package definition |
+| `packaging/archlinux` | Arch Linux `PKGBUILD` |
 
 ## Contributing