Update BuildStream's documentation and website with an update on the Release Policy change

Background

From #901 (closed); basically:

• 1.2.x will not be actively maintained long term
• new version will be 2.0 which will break api

Task description

• Update website - website!118 (merged)
• Update README
• Blog post - https://blogs.gnome.org/tvb/2019/03/04/buildstream-news-and-2-0-planning/

Acceptance Criteria

All is updated