Document incompatibilities of the 2.0 release

These are provisional and extracted from https://mail.gnome.org/archives/buildstream-list/2018-October/msg00080.html:

• bst shell --sysroot semantics now work with a simple sysroot directory instead of our build directory format, which no longer litters the user's disk space (buildstream#539 (closed))
• A fair amount of breakage in the workspace related commands because Workspace UX reworking (buildstream#695 (closed))
• The bst source-bundle command has been removed and it has become a feature of bst source-checkout (buildstream#672 (closed))
• Default implementation of "strip-commands" has been removed (buildstream#304 (closed))
• MAKEFLAGS & V environment settings have been removed from the manual element (buildstream!916 (merged))
• Changes in the non-interactive prompt behaviour (buildstream#744 (closed))
• git plugin does not checkout submodules by default anymore (buildstream#783) This seems is not going to happen anymore
• The git plugin does not create a local .git repository by default anymore. See buildstream!906 (merged) and buildstream#487 (closed)
• bst track and bst fetch commands are now obsolete. Their functionality is provided by bst source track and bst source fetch respectively.(See buildstream!1086 (merged))

For 1.2 incompatibilities see: buildstream#226 (closed)

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Added a link to buildstream!916 (merged), which also handily shows that it's been merged.

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mentioned in merge request buildstream!972

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mentioned in issue buildstream#783

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