explain the Debian package file release in document and contribution …

…guide

CONTRIBUTING.md

@@ -66,6 +66,30 @@ git add testdata/snapshots
 These snapshot tests were added because `clap`'s major changes caused some regressions. `clap`'s major version bump caused API
 breaking changes frequently and it was hard to follow them without tests.
 
+## How to generate `.deb` file
+
+[cargo-deb][] needs to be installed. The `.deb` package metadata is described in `[package.metadata.deb]` section in
+[Cargo.toml](./Cargo.toml).
+
+The `.deb` file contains a manual file for `man` and a Bash completion file. You need to generate them in advance.
+
+```sh
+cargo build --release
+# Create a manual file
+target/release/hgrep --generate-man-page > target/release/hgrep.1
+# Create a Bash completion file
+target/release/hgrep --generate-completion-script bash > target/release/hgrep.bash
+# Generate `.deb` file at `target/debian/`
+cargo deb
+# Try to install the package
+sudo dpkg -i target/debian/hgrep_*.deb
+```
+
+These build steps should be done on Linux. I haven't tried cross compilation.
+
+The release CI workflow automatically builds `.deb` file and uploads it to [the GitHub releases page][releases] so usually you
+don't need to build it manually.
+
 ## Make a new release
 
 Let's say we're releasing v1.2.3.
@@ -102,6 +126,7 @@ Syntax set and theme set are managed in [./assets](./assets) directory. See [the
 [clippy]: https://github.com/rust-lang/rust-clippy
 [rustfmt]: https://github.com/rust-lang/rustfmt
 [repo]: https://github.com/rhysd/hgrep
+[cargo-deb]: https://github.com/kornelski/cargo-deb
 [release-ci]: ./.github/workflows/release.yml
 [releases]: https://github.com/rhysd/hgrep/releases
 [changelog-from-release]: https://github.com/rhysd/changelog-from-release

README.md

@@ -84,6 +84,17 @@ cd /usr/pkgsrc/textproc/hgrep
 make install
 ```
 
+### Via [APT][apt] package manager on Debian or Ubuntu
+
+Visit [the releases page][releases] and download `.deb` package file. It can be installed via `dpkg` command:
+
+```sh
+sudo dpkg -i hgrep_v0.3.5_amd64.deb
+```
+
+The manual (for `man` command) and Bash completion is automatically installed. If you're using some other shell, setup the shell
+completion by yourself. See ['Generate completion scripts' section](#gen-completion-scripts) for more details.
+
 ### Via [cargo][] package manager
 
 ```sh
@@ -354,6 +365,8 @@ $Env:HGREP_DEFAULT_OPTS = "--glob '!C:\Program Files'"
 
 See `--help` for the full list of available options in your environment.
 
+
+<a name="gen-completion-scripts"></a>
 ### Generate completion scripts
 
 Shell completion script for `hgrep` command is available. `--generate-completion-script` option generates completion script and
@@ -493,6 +506,7 @@ hgrep is distributed under [the MIT license](./LICENSE.txt).
 [nushell]: https://www.nushell.sh/
 [homebrew]: https://brew.sh/
 [macports]: https://www.macports.org/
+[apt]: https://www.debian.org/doc/manuals/debian-faq/pkgtools.html
 [new-issue]: https://github.com/rhysd/hgrep/issues/new
 [syntect]: https://github.com/trishume/syntect
 [bat-vs-syntect]: #bat-printer-vs-syntect-printer