*: Add LEARNERS row to information_schema.placement_policies in d…

…esign doc (pingcap#31700)

ref pingcap#18030

docs/design/2020-06-24-placement-rules-in-sql.md

@@ -170,6 +170,7 @@ A new system table `information_schema.placement_policies` is added to view all
 | LEARNER_CONSTRAINTS  | varchar(1024) | YES  |      | NULL    |       |
 | SCHEDULE             | varchar(20)   | YES  |      | NULL    |       |
 | FOLLOWERS            | bigint(64)    | YES  |      | NULL    |       |
+| LEARNERS             | bigint(64)    | YES  |      | NULL    |       |
 +----------------------+---------------+------+------+---------+-------+
 ```