Add tldr entry for buildstream

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1357)
In GitLab by [[Gitlab user @tcanabrava]](https://gitlab.com/tcanabrava) on Jul 8, 2020, 11:45

tldr is a `man like` application that focus on most common options ignoring specifics, it's quite userful when you want to have a `tldr` version of a manual, for instance, compare the output of `man zip` to `tldr zip`, the man is 5 pages, the tldr is half a page, and both of them gives basically the same information.

Start with the linked GitLab issue and review how existing tldr entries are organized. Determine the expected BuildStream command examples and options, then add the entry in the repository's established tldr location and verify that it matches the project's documentation conventions.

Written by the indexing model from the issue text.