Add fish autocompletion support

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/557)
In GitLab by [[Gitlab user @Qinusty]](https://gitlab.com/Qinusty) on Aug 6, 2018, 11:48

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)

BuildStream should provide fish autocompletion instructions similar to those for [bash](http://buildstream.gitlab.io/buildstream/install_linux_distro.html#bash-completions)

## Task description

[//]: # (Short summary of the action to be executed)

* [ ] Provide autocompletion setup instructions for fish

## Acceptance Criteria

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )

The documentation should provide fish users with enough guidance to have custom autocompletion for buildstream.

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Start with the existing Bash completions guidance linked in the issue and inspect the surrounding BuildStream installation documentation. Add equivalent fish setup instructions, then verify that the documented steps provide BuildStream command completion for fish users.

Written by the indexing model from the issue text.