Add BuildStream to Linguist: https://github.com/github/linguist

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1351)
In GitLab by [[Gitlab user @frazerclews]](https://gitlab.com/frazerclews) on Jul 6, 2020, 19:28

## Background

[//]: # BuildStream doesn't show up in GitLab, GitHub, etc repositories which doesn't help it being well known. And would help those getting into a BuildStream project a better understanding of the project. Since BitBake is in the linguist list, I am sure BuildStream can be too

## Task description

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

* [ ] Add BuildStream following linguists steps https://github.com/github/linguist/blob/master/CONTRIBUTING.md#adding-a-language
* [ ] Use YAML as the grammar for BuildStream

## Acceptance Criteria

[//]: # Having BuildStream show up along side other languages in GitHub, GitLab, etc projects, and hopefully have BuildStream have grammatical highlighting if we use the YAML grammar already in linguist

Start with github/linguist's CONTRIBUTING.md section on adding a language and follow its documented entry points for BuildStream. Use the YAML grammar as specified, then verify that BuildStream appears as a recognized language with grammatical highlighting in supported repositories.

Written by the indexing model from the issue text.