Re-add map reduce deprecation

Author: jeff-allen-mongo

source/release-notes/5.0-compatibility.txt

@@ -287,6 +287,16 @@ When run against :binary:`~bin.mongos`, the ``shards`` field in the
 collection on a particular shard. Size values in the ``shards`` field
 are expressed as integers.
 
+Map-Reduce
+----------
+
+Starting in version 5.0, MongoDB deprecates the
+:doc:`map-reduce</core/map-reduce>` operation.
+
+For examples of aggregation pipeline alternatives to map-reduce
+operations, see :doc:`/reference/map-reduce-to-aggregation-pipeline` and
+:doc:`/tutorial/map-reduce-examples`.
+
 Auditing
 --------