Update description to give hints about stacking possibilities
Update description to give hints about stacking possibilities

It is now expected that several implementations of scope() may be used together
to combine a set of features, for example: define in private scope and run only
when all dependencies are ready.

Besides, the scope-bootstrap is now intended to be used for the definition of
other implementations of scope() in the global context.

eric-brechemier/scopeornot,eric-brechemier/scopeornot