Mention scope expansion in incubation requirements

The incubation requirements mention that a project should have a clearly
defined and non-overlapping scope, but we don't make any mention about
our cautious approach to widening the scope of OpenStack.

Brought up here:

  http://lists.openstack.org/pipermail/openstack-dev/2013-December/022412.html

Change-Id: I31c18a0074224b973cb44373993d3e7e20a7d085
Merge "Mention scope expansion in incubation requirements"