Add anti-deadlock instructions to DB update README (#1552)

db/README.md

@@ -56,6 +56,12 @@ following steps:
     table statements can be used as is, whereas alter table statements should be
     written to change any existing tables.
     
+    Note that each incremental file MUST be limited to changes to a single
+    table (otherwise it may hit deadlock when applying on sandbox/production
+    where it'll be competing against live traffic that may also be locking said
+    tables but in a different order). It's OK to include these separate Flyway
+    scripts in a single PR.
+
     This script should be stored in a new file in the
     `db/src/main/resources/sql/flyway` folder using the naming pattern
     `V{id}__{description text}.sql`, where `{id}` is the next highest number