Provide installation options on Linux and BSD

[mattt]

We should provide a convenient way to install the dependabot on Linux platforms.

[mattt]

The gh CLI does a good job of making itself available to different packaging systems, so we should follow their example: https://github.com/cli/cli/blob/trunk/docs/install_linux.md

[jeffwidman]

I realize it'd be convenient to support those upstream package managers, but I don't foresee us officially supporting those anytime in the foreseeable future... This is a go-based program, a simple go install is trivial, and anyone running Linux/BSD (or even playing with :dependabot:!) should be very comfortable following our existing install instructions.

This is open source, so anyone who wants is welcome to drive unofficial installation methods with those ecosystems package managers.

I'm going to close, as I just don't think there's a clear next action related to this repo here.