Added idea to warn if systems don't have command-flushing constraint

Author: alice-i-cecile

rfcs/45-stageless.md

@@ -563,4 +563,5 @@ Despite the large scope of this RFC, it leaves quite a bit of interesting follow
    2. How does this work for run criteria that are not locally defined?
 6. A more cohesive look at plugin definition and configuration strategies.
 7. A graph-based system ordering API for dense, complex dependencies.
-8. Store systems in the `World` as entities.
+8. Store systems in the `World` as entities?
+9. Warn if systems that emit commands do not have an appropriate command-flushing ordering constraint.