Install instructions re plugins and plugin-dependencies is out of date

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1374)
In GitLab by [[Gitlab user @DouglasWinship]](https://gitlab.com/DouglasWinship) on Jul 28, 2020, 15:54

The following discussion from !1999 should be addressed:

- [ ] [[Gitlab user @BenjaminSchubert]](https://gitlab.com/BenjaminSchubert) started a [discussion](https://gitlab.com/BuildStream/buildstream/-/merge_requests/1999#note_385937591): (+1 comment)

> None of those plugins but `tar` are in Core on master, I don't think they should be listed there.
>
> I am also not sure how to best keep this up to date. I'd rather just mention that some plugins might have extra deps and that it will be check at runtime?

In addition, I think the install instructions need to have more explicit mention of the external plugin directory(s)

We explicitly advertise BuildStream as being compatible with build-systems like cmake and autotools, so a user who has installed BuildStream, will probably expect these plugins to be available, and the install should explain how to get them.

Start with the install instructions and review the discussion linked from merge request 1999 to identify which plugins are in Core. Update the documentation to distinguish Core and external plugins, explain plugin directories and extra dependencies, and describe how users can obtain the CMake and autotools plugins.

Written by the indexing model from the issue text.