Install instructions re plugins and plugin-dependencies is out of date

[Cynical-Optimist]

See original issue on GitLab
In GitLab by [Gitlab user @DouglasWinship] on Jul 28, 2020, 15:54

The following discussion from !1999 should be addressed:

• [Gitlab user @BenjaminSchubert] started a discussion: (+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.