Note known bug in job preemption code

NEWS

@@ -8,6 +8,9 @@ documents those changes that are of interest to users and admins.
     sched/backfill plugin. Added configuration parameter:
     PreemptMode=off|suspend|kill|requeue|checkpoint 
     to enable/disable the job preemption logic (disabled by default).
+    (NOTE: There are some problems with memory management which could prevent a 
+    job from starting when memory would be freed by a job being requeued or 
+    otherwise removed)
  -- If the --partition option is used with the sinfo or squeue command then
     print information about even hidden partitions.
  -- Replaced misc cpu allocation members in job_info_t with select_job_res_t