Cover use of dedicated project areas for patching components

We state that authors of submissions to extend existing components could
be asked to submit the change upstream, however we do not cover the option
of using a dedicated project area for tweaked components should changes
not be of general applicability to Apertis.

Highlight that dedicated project areas may be made available for this
purpose.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>