Add Linux links to README

Problem to solve

The README suggests Homebrew as the only supported installation method on Linux, however, glab exists as a snap package and in many distros' repositories.

Let's mention these as "community-maintained" to suggest we don't offer support for these installation methods. Let's also link to https://repology.org/project/glab-gitlab-cli/versions with an image:

[![Packaging status](https://repology.org/badge/vertical-allrepos/glab-gitlab-cli.svg)](https://repology.org/project/glab-gitlab-cli/versions)

Proposal

Further details

Links / references