[saimskywalker]

Closes #1018

People keep asking how to install gitui on Debian/Ubuntu and posting install scripts in that issue, so this adds a short "Debian / Ubuntu" entry to the install instructions in the README, next to the other Linux distros.

It changes the following:

• README: explains that gitui is not in the official Debian/Ubuntu repositories yet (there is an open ITP, https://bugs.debian.org/1041509), and lists the two ways that work today: the statically linked x86_64 release binary (with the ARM tarball names and their glibc requirement), and cargo install gitui --locked with build-essential plus rustup
• CHANGELOG: entry under Unreleased / Added

No install script, just three plain commands that use the releases/latest/download/<asset> URL, so the snippet doesn't need updating on every release.

What I checked:

• packages.debian.org (all suites), sources.debian.org, tracker.debian.org, and Launchpad's published sources for the Ubuntu primary archive: no gitui package anywhere. The ITP from 2023 is still open.
• The v0.28.1 release assets: gitui-linux-x86_64.tar.gz is static-pie linked (musl). gitui-linux-aarch64.tar.gz and gitui-linux-armv7.tar.gz are dynamically linked against glibc, and the newest symbol version they need is GLIBC_2.28, so Debian 10 and Ubuntu 20.04 or later. armv7 is the one that matches Debian/Ubuntu armhf; gitui-linux-arm targets ARMv6.
• Ran the documented download/extract/install steps against the current release.
• Building from source: the default vendor-openssl feature builds OpenSSL, libgit2 and libssh2 from source with cc, so the build needs a C compiler, make and perl (all pulled in by build-essential). cmake isn't needed. The distro rustc is older than the 1.88 MSRV (Debian 13 ships 1.85, Ubuntu 24.04 ships 1.75), which is why the README points to rustup.

I left out the snap (gitui on the Snap Store) because it's published by a third party and not verified, but I can add it if you'd like it listed.

I followed the checklist:

• n/a I added unittests (docs only)
• n/a I ran make check without errors (docs only)
• n/a I tested the overall application
• I added an appropriate item to the changelog