Document incompatibilities of the 1.4 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 (#539 (closed))
• A fair amount of breakage in the workspace related commands because Workspace UX reworking (#695 (closed))
• The bst source-bundle command has been removed and it has become a feature of bst source-checkout (#672 (closed))
• Default implementation of "strip-commands" has been removed (#304 (closed))
• MAKEFLAGS & V environment settings have been removed from the manual element (!916 (merged))
• Changes in the non-interactive prompt behaviour (#744 (closed))

For 1.2 incompatibilities see: #226 (closed)